













































































































































































































































V 




























A SYSTEM 


OK 


j i *» 
\J fu; w. 


3gpr 

NATURAL PHILOSOPHY: 


IN WHICH THK 


PRINCIPLES OF MECHANICS, 

HYDROSTATICS, hydraulics, pneumatics, acoustics, optics, 

ASTRONOMY, ELECTRICITY, MAGNETISM, STEAM ENGINE, 
ELECTRO-MAGNETISM, ELECTROTYPE, PHOTO¬ 
GRAPHY, AND DAGUERREOTYPE, 

ARE 

FAMILIARLY EXPLAINED, ..... 

AND ILLUSTRATED BY 

MORE THAN TWO HUNDRED AND FIFTY ENGRAVINGS; 

TO WHICH ARE ADDED, 

QUESTIONS FOR THE EXAMINATION OF PUPILS. 

DESIGNED FOR 

THE USE OF SCHOOLS AND ACADEMIES. 


BY J. L. COMSTOCK, M. D. 

AUTHOR of introduction to mineralogy, elements of chemistry 

INTRODUCTION TO BOTANY, OUTLINES OF GEOLOGY, OUTLINES 
OF PHYSIOLOGY, NATURAL HISTORY OF BIRDS. &C. 


_ * i 5 ' - -•* " • V 

SEVENTY-NINTH EDITION. 

’> ) > 

—- 

* ) * , 

NEW-YORK: 

PRATT, WOODFORD AND CO., 






Entered, 

According to act of Congress, in the year 1844, b) 

J. L. COMSTOCK, 

in the Clerk’s Office of the District Court of Connecti ,.»x. 


Urv>v • ^ ( WV\ ) \«yvu 


Thn \ 5 “ * 1 \ 


STEREOTYPED BY PRINTED BY 

RICHARD H, HOBBS, CA&3!, TIFFANY & BURNHAM 
HARTFORD, CONN. 


HARTFORD, CONN. • 









ADVERTISEMENT. 


The publishers being again under the necessity of 
having a new set of stereotype plates cast for this work, 
the Author has taken this opportunity of making such 
additions and improvements as to make the work cor¬ 
respond with the present state of the Arts and Sciences, 
so far as they come within the scope of a School Book. 

The whole has been carefully reviewed, and new mat¬ 
ter has been added wherever it was thought the book 
could thereby be improved. 

The Author has intended to embrace every thing 
proper for his book, which has been invented, or brought 
forward since the last stereotype plates were cast. 

The new matter covers more than 30 pages, and em¬ 
braces the subjects of Water Wheels, Gunnery, Elec¬ 
trotype, showing the manner of gilding, silvering, an 1 
making copper casts, Photography, Daguerreotype, Rus¬ 
sel’s Planetarium, Morse’s Electro-Magnetic Telegraph, 
Horse Power, &c. 

Hartford , August, 1844. 



P RE FACIE. 


While we have recent and improved systems, of Geogra 
phy, of Arithmetic, and of Grammar, in ample variety,—and 
Reading and Spelling Books in corresponding abundance, 
many of which show our advancement in the science of edu¬ 
cation, no one has offered to the public, for the use of our 
schools, any new or improved system of Natural Philoaopny. 
And yet this is a branch of education very extensively studied 
at the present time, and probably would be much more so, 
were some of its parts so explained and illustrated as to make 
them more easily understood. 

The author therefore undertook the following work at the 
suggestion of several eminent teachers, who for years have 
regretted the want of a book on this subject, more familiar 
in its explanations, and more ample in its details, than any 
now in common use. 

The Conversations on Natural Philosophy, a foreign work, 
now extensively used in schools, though beautifully written, 
and often highly interesting, is, on the whole, considered by 
most instructors as exceedingly deficient—particularly in 
wanting such a method in its explanations, as to convey to 
the mind of the pupil precise and definite ideas; and also in 
the omission of many subjects, in themselves most useful to 
the student, and at the same time most easily taught. 

It is also doubted by many instructors, whether Conversa¬ 
tions is the best form for a book of instruction, and particu¬ 
larly on the several subjects embraced in a system of Natu¬ 
ral Philosophy. Indeed, those who have had most experi¬ 
ence as teachers, are decidedly of the opinion that it is not; 
and hence, we learn, that in those parts of Europe where the 
subject of education has received the most attention, and 
consequently, where the best methods of conveying instruc¬ 
tion are supposed to have been adopted, school books, in the 
form of conversations, are at present entirely < ut of use. 



VI 


PREFACE. 


The author of the following system hopes to have illus¬ 
trated and explained most subjects treated of, in a manner . 
so familiar as to be understood by the pupil, without requir¬ 
ing additional diagrams, or new modes of explanations from 
the teacher. 

Every one who has attempted to make himself master of 
a difficult proposition by means of diagrams, knows that the 
great number of letters of reference with which they are 
sometimes loaded, is often the most perplexing part of the 
subject, and particularly when one figure is made to answer 
several purposes, and is placed at a distance from the expla¬ 
nation. To avoid this difficulty, the author has introduced 
additional figures to illustrate the different parts of the sub¬ 
ject, instead of referring back to former ones, so that the stu¬ 
dent is never perplexed with many letters on any one figure. 
The figures are also placed under the eye, and in immediate 
connection with their descriptions, so that the letters of refer¬ 
ence in the text, and those on the diagrams, can be seen at 
the same time. In respect to the language employed, it has 
been the chief object of the author to make himself under¬ 
stood by those who know nothing of mathematics, and who 
indeed had no previous knowledge of Natural Philosophy. 
Terms of science have therefore been as much as possible 
avoided, and when used, are explained in connection with the 
subjects to which they belong, and, it is hoped, to the com¬ 
prehension of common readers. This method was thought 
preferable to that of adding a glossary of scientific terms. 

The author has also endeavored to illustrate the subjects 
as much as possible by means of common occurrences, or 
common things, and in this manner to bring philosophical 
truths as much as practicable within ordinary acquirements. 

It is hoped, therefore, that the practical mechanic may take 
some useful hints concerning his business, from several parts 
of the work. 


INDEX 


Page. 

Ascent ot bodies,.34 

Actiou and reaction, . . . . 39 

Air, elasticity of,. . 133 

expansion of,.134 

compression,.134 

Alarm bell,.357 

Air-gun,.133 

pump,.136 

Atmosphere, pressure of,.135 

phenomena of, ... . 178 

Attraction, in general,.14 

of cohesion,.15 

of gravitation, .... 16 

capillary,.17 

magnetic,.20 

electrical,.21 

in proportion to matter, . 32 

Astronomy, . . '.240 

Archimedes’screw, ....... 125 

Asteroids,.. . 253 

Atwoods’ machine, ....... 27 

Axis of a planet,.242 

Bramahs’ press,.108 

Balance,.114 

Barker’s mill, . . 129 

Bodies, properties of,.9 

fall of light,.35 

ascending,. . 34 

Boats, men pulling,.33 

Battery, galvanic,.343 

Barometer,.140 

construction of,.143 

water,.144 

wheel,.145 

use of,.146 

Brittleness,.22 

Burning Glass,.209 

Capstan,.79 

Casts copied,.346 

Cannon ball,.57 

Ceres, .... 253 

Centrifugal force,.262 

Centripetal force,.262 

Camera obscura,.228 

Comets,. ... 312 

Chromatics.231 

Coal, power of,.170 

Convex lens,.207 

Condenser,.139 

Constellation,. 245 

Cup and shilling,.186 

Colors of objects,. * . 237 

Daguerreotype,.353 

Day and night,.271 

Decomposition,.12 

Density,.18 

of the planets,.246 

Divisibility,.11 

Ductility.23 


Earth,. 251—265 

circles and divisions of, . . . 267 
distance from the Sun, . . 266 


Earth, falling to the Sun,. . . . 31 

Ecliptic,. 244—269 

Eclipses,.295 

solar,.299 

lunar,.298 

Electro-magnetism,.334 

laws of,.337 

Electricity,. ..314 

Electrical machine,.318 

battery,.327 

telegraph,.358 

Electroscope,.315 

Electrotype,. 344 

Electrometer,.322 

Electro-gilding,.348 

Electro plating, . ..350 

Equation of time,.285 

Equinoxes,.274 

Equilibrium,. 54 

Extension,.10 

Falling bodies,.26 

Fire engine,.151 

Figure,.10 

Fluids, discharge of,.123 

Focal distance,.207 

Fusible metal,.346 

Force not created,.84 

what,.69 

of gravity,.24 

Galvanism,.333 

Galvanic battery,.343 

Globular form,.16 

Gold leaf,. 11 

Governor.165 

Gravity, force of,.24 

specific, . 113 

not diminished,. 57 

Gymnotus electricus, ..327 

Gunnery,.60 

Hay, load of,.51 

Hardness,.21 

Herschel, . •.258 

High-pressure engine,.167 

Hiero’s fountain,.152 

Harp, aeolian, ..177 

Horse power,.168 

Horizon,.268 

Hydrostatics,.101 

Hydrostatic bellows,.107 

Hydrometer,.116 

Hydrophane,.240 

Hydraulics.119 

Hydrostatic piress,.107 

Impenetrability,.9 

Inertia,.12 

centre of,.53 

Inclined plane,.91 

Juno,.253 

Jupiter,.254 

Latitude and longitude,.303 

how found,.30ft 

Lenses.206 


















































































































VIH 


INDEX. 


Lens concave,. 

convex, . 

Lever,. 

compound,. 

Level, water,. 

Lightning rods, . ... . • 
Light, refraction of,. . . . 

reflection of, ... . 
Longitude, ....... 

how found, . . . 

Mars,.. 

Magic lantern, .... . 

Magnetism,. 

electro, . . - • 
Matter, inertia of, 

Malleability, . 

Magnetic needle, . > . • . 
Mechanics, . ... . . . 

Metronome,. 

Mercury, .. 

Microscope,. 

compound, . . « 
solar, » - . •• • 
Momentum, ....... 

Mechanical powers,. . . . 

Mirrors,. 

convex, . » 

concave, . 

plane,. 

metallic,. 

Moon, . . .. 

falling to the Earth, . . 
phases of, . . . . . 

surface of, .... . 

Motion defined,. 

absolute and relative, 
velocity of, ... . 

reflected,. 

compound, . . . . 

circular, . . . . . 

crank,. 

curvilinear, . . . . 

resultant, . . . » . 

Morse’s telegraph, . . . . 

Musical strings, .... 

instruments, . . • 

Musk, scent of,. 

Optics, ........ 

definitions in, . . . 
Optical instruments, . . 

Orbit, what,.. 

Pallas,.. 

Plaster of paris casts, . . , 

Planatarium. 

Planets,. 

density of, . . 
situation of, . 
motions of, 

Pendulum, . 

gridiron, . . . 

Penumbra,. 

Photography,. 

Pile-driver,. 

Power varying, .... 
Perkins’ experiments, . . 
Prismatic spectrum,. . . 
Properties of bodies, . . 

Pneumatics,. 

Pomps,. 

common, .... 
lifting. 


Page. 

. . 210 
. 207 

. . 70 
. . 76 
*. . Ill 
. . 326 
. . 185 
. . 188 
. . 303 
. . 305 
. . 252 
. . 230 
. . 328 
. . 334 
. . 13 
. . 23 
. . 331 
. . 68 
. . 67 
. . 250 
. . 220 
, . 221 
.. . 222 
. . 38 
-. . 98 
. . 184 
. . 192 
. . 186 
. . 190 
. . 204 
252—290 
. . 31 
. . 292 

. . 294 
. . 36 
. . 37 
. . 37 
. . 41 
. . 42 
. . 44 
. . 163 
. . 55 
. . 62 
. . 358 
. . 177 
. . 176 
. . 11 
. . 182 
. . 184 
. . 220 
. . 242 
. . 253 
. . 345 
. . 309 
. . 241 
. . 246 
. . 259 
. . 260 
. . 64 
. . 65 
. . 299 
. . 351 
. . 39 
. . 81 
. . 102 
. . 231 
. . 9 

. . 132 
141-148 
. . 148 
. . 147 


Pump, forcing,. 

Pulley,. 

Whites’,. 

Rain guage,. 

Rain,. 

Rainbow,. 

Rarity,.. 

Revolution of two wheels, . . 
Rockets, how moved, .... 
Reflection by lenses, .... 
Refraction by lenses, .... 

Retina,.. . 

Rotation of a wheel, .... 

Saturn, . 

Scales,.. 

Seasons, . 

heat and cold of, . . . 

Screw,. 

perpetual,. 

Archimedes’,. 

Sound, propagation of, ... . 

reflection of,. 

Solstices,.. 

Summer and winter, .... 
Spring, intermitting, .... 

Solar system. 

Steelyards,. 

Solar and siderial time, . . . 

Stars, fixed,. 

Steam engine,. 

Savary’s,. 

Newcomen’s,.... 

Watt’s,. 

low and high pressure, 

Sun,. 

Syphon, . 

Temporary magnets, .... 

Telescope,. 

reflecting,. 

refraction, .... 

Tenacity,. 

of wood,. 

of metals,. 

Tides, ......... 

Torpedo,. 

Umbra,. 

Velocity of falling bodies, . . . 

of a ball. 

Venus,. 

Vision,. 

perfect,. 

imperfect,. 

angle of,. 

Vesta,. 

Vibration of a wire, . . . . 

Wedge,. 

Wheel and axle,. 

Windlass,. 

Water, elasticity of, . . . . 
equal pressure of, . . . 
bursting power of, . . 
raised by ropes, . . . 

running,. 

machines for raising, 

wheels,. 

Wood, composition of, . . . . 
Whispering gallery, . . . . 

Wind,. 

trade,. 

Zodiac,. 


f*gc. 
. 149 
. 85 
. 90 
. 182 
. 181 
. 233 
. 21 
. 341 
. 19 
. 210 
. 206 
. 219 
. 339 
. 256 
. 72 


272 

277 

94 

97 


. . 125 
. . 171 
. . 174 
. . 273 
. . 279 
. . 118 
. . 241 
. . 73 
. . 283 
. . 307 
. . 152 
. . 153 
. . 156 
. . 159 
165—167 
. . 247 
. . 117 
. . 342 
. . 223 
. . 226 
. . 223 
. . 23 
. . 23 
. . 24 
. . 300 
. . 227 
. . 299 
. . 26 
. . 60 
. . 250 
. . 211 
. . 215 
. . 216 


. 217 
. 253 
. 337 
. 93 
. 77 


. 79 


. . 112 
. . 103 
. . 106 
. . 128 
. . 125 
. . 125 
. . 130 
. . 12 
. . 175 
. . 178 
. . 179 
. . 944 







































































































































NATURAL PHILOSOPHY. 


THE PROPERTIES OF BODIES. 

». A Body is any substance of which we can gain a know¬ 
ledge by our senses. Hence air, water, and earth, in all theii 
modifications, are called bodies. 

2. There are certain properties which are common to all 
bodies. These are called the essential properties of bodies. 
They are Impenetrability, Extension, Figure, Divisibility , 
Inertia , and Attraction. 

3. Impenetrability. —By impenetrability, it is meant that 
two bodies cannot occupy the same space at the same time, 
or, that the ultimate particles of matter cannot be penetrated. 
Thus, if a vessel be exactly filled with water, and a stone, 
or any other substance heavier than water, be dropped into 
it, a quantity of water will overflow, just equal to the size of 
the heavy body. This shows that the stone only separates 
or displaces the particles of water, and therefore that the two 
substances cannot exist in the same place at the same time. 
If a glass tube open at the bottom, and closed with the thumb 
at the top, be pressed down into a vessel of water, the liquid 
will not rise up and fill the tube, because the air already in 
the tube resists it; but if the thumb be removed, so that the 
air can pass out, the water will instantly rise as high on 
the inside of the tube as it is on the outside. This shows 
that the air is impenetrable to the water. 

4. If a nail be driven into a board, in common language, it 
is said to penetrate the wood, but in the language of philoso¬ 
phy it only separates, or displaces the particles of the wood. 


What is a body ? Mention several bodies. Whnt are the essential pro- 
nerties of bodies ? What is meant by impenetrability ? How is it proved that 
air and water are impenetrable ? When a nail is driven into a board or piece 
of lead, are the particles of these bodies penetrated or separated! 






ic 


PROPERTIES OF BODIES. 


The same is the case, if the nail be driven into a piece of 
lead; the particles of the lead are separated from each other, 
and crowded together, to make room for the harder body, 
but the particles themselves are by no means penetrated by 
the nail. 

5. When a piece of gold is dissolved in an acid, the par¬ 
ticles of the metal are divided, or separated from each other, 
and diffused in the fluid, but the particles of gold are sup¬ 
posed still to be entire, for if the acid be removed, we obtain 
the gold again in its solid form, just as though its particles 
had never been separated. 

6. Extension. —Every body, however small, must have 
length, breadth, and thickness, since no substance can exist 
without them. By extension, therefore, is only meant these 
qualities. Extension has no respect to the size, or shape of a 
body. 

7. The size and shape of a block of wood a foot square 
is quite different from that of a walking stick. But they 
both equally possess length, breadth, and thickness, since 
the stick might be cut into little blocks, exactly resembling 
in shape the large one. And these little cubes might again 
be divided until they were only the hundredth part of an inch 
in diameter, and still it is obvious, that they would possess 
length, breadth, and thickness, for they could yet be seen, 
felt, and measured. But suppose each of these little blocks 
to be again divided a thousand times, it is true we could not 
measure them, but still they would possess the quality of ex¬ 
tension, as really as they did before division, the only differ¬ 
ence being in respect to dimensions. 

8. Figure or form is the result of extension, for we cannot 
conceive that a body has length and breadth, without its also 
having some kind of figure, however irregular. 

9. Some solid bodies have certain or determinate forms 
which are produced by nature, and are always the same 
wherever they are found. Thus, a crystal of quartz has six 
sides, while a garnet has twelve sides, these numbers being 
invariable. Some solids are so irregular, that they cannot 
be compared with any mathematical figure. This is the 
case with the fragments of a broken rock, chips of wood 
fractured glass, &c. 


Are the particles of gold dissolved, or only separated, by the acid ? What 
is meant by extension ? In how many directions do bodies possess extension? 
Of what ^figure, or form, the result? Do all bodies possess figure ? Wluu 
solids are regular m their forms ? - * '• ■ & u 


What bodies are irregular ? 



PROPERTIES OF BODIES. 


11 


10. Fluid bodies have no determinate forms, but take their 
shapes from the vessels in which they happen to be placed. 

11. Divisibility.— By the divisibility of matter, we mean 
that a body may be divided into parts, and that these parts 
may again be divided into other parts. 

12. It is quite obvious, that if we break a piece of marble 
into two parts, these two parts may again be divided, and 
that the process of division may be continued until these 
parts are so small as not individually to be seen or felt. But 
as every body, however small, must possess extension and 
form, so we can conceive of none so minute but that it may 
again be divided. There is, however, possibly a limit, beyond 
which bodies cannot be actually divided, for there may be 
reason to believe that the atoms of matter are indivisible by 
any means in our power. But under what circumstances 
this takes place, or whether it is in the power of man during 
his whole life, to pulverize any substance so finely, that it 
may not again be broken, is unknown. 

13. We can conceive, in some degree, how minute must 
be the particles of matter, from circumstances that every day 
come within our knowledge. 

14. A single grain of musk will scent a room for years, 
and still lose no appreciable part of its weight. Here, the 
particles of musk must be floating in the air of every part 
of the room, otherwise they could not be every where per¬ 
ceived. 

15. Gold is hammered so thin, as to take 282,000 leaves 
to make an inch in thickness. Here, the particles still ad¬ 
here to each other, notwithstanding the great surface which 
they cover,—a single grain being sufficient to extend over a 
surface of fifty square inches. 

16. The ultimate particles of matter, however widely they 
may be diffused, are not individually destroyed, or lost, but 
under certain circumstances, may again be collected into a 
body without change of form. Mercury, water, and many 
other substances, may be converted into vapor, or distilled in 
close vessels, without any of their particles being lost. In 
such cases, there is no decomposition of the substances, but 


What is meant by divisibility of matter? Is there any limit to the divisi¬ 
bility of matter? Are the atoms of matter divisible? What examples are 
given of the divisibility of matter ? How many leaves of gold does it take to 
make an inch in thickness ? How many square inches may a grain of gold be 
made to cover? Under what circumstances may the particles of matter again 
be collected in their original form ? 



PROPERTIES OF BODIES 


VI 

only a change of form by the heat, and hence the mercury 
and water assume their original state again on cooling. 

17. When bodies suffer decomposition or decay, their ele¬ 
mentary particles, in like manner, are neither destroyed nor 
lost, but only enter into new arrangements or combinations 
with other bodies. 

18. When a piece of wood is heated in a close vessel, such 
as a retort, we obtain water, an acid, several kinds of gas, 
and there remains a black, porous substance, called charcoal. 
The wood is thus decomposed, or destroyed, and its particles 
take a new arrangement, and assume new forms, but that 
nothing is lost is proved by the fact, that if the water, acid, 
gasses, and charcoal, be collected and weighed, they will be 
found exactly as heavy as the wood was before distillation. 

19. Bones, flesh, or any animal substance, may in the 
same manner be made to assume new forms, without losing 
a particle of the matter which they originally contained. 

20. The decay of animal or vegetable bodies in the open 
air, or in the ground, is only a process by which the particles 
of which they were composed, change their places and as¬ 
sume new forms. 

21. The decay and decomposition of animals and vegeta¬ 
bles on the surface of the earth form the soil, which nour¬ 
ishes the growth of plants and other vegetables ; and these, 
in their turn, form the nutriment of animals. Thus is there 
a perpetual change from death to life, and from life to death, 
and as constant a succession in the forms and places, which 
the particles of matter assume. Nothing is lost, and not a 
particle of matter is struck out of existence. The same mat¬ 
ter of which every living animal, and every vegetable was 
formed, before and since the flood, is still in existence. As 
nothing is lost or annihilated, so it is probable that nothing 
has been added, and that we, ourselves, are composed of par¬ 
ticles of matter as old as the creation. In time, we must, in 
our turn, suffer decomposition, as all forms have done before 
us, and thus resign the matter of which wc are composed, to 
form new existences. 

22. Inertia. —Inertia means passiveness or want of power. 
Thus matter is, of itself, equally incapable of putting itself in 
motion, or of bringing itself to rest when in motion. 


When bodies suffer decay, are their particles lost ? What becomes of the 
particles of bodies which decay? Is it probable that any matter has been an¬ 
nihilated or added, since the first creation ? What is said of the particles of 
matter of which we are made ? What does inertia mean ? 



PROPERTIES OF BODIES. 


13 


23. It is plain that a rock on the surface of the earth 
never changes its position in respect to other things on the 
eaith. It has of itself no power to move, and would, there¬ 
fore, for ever lie still, unless moved by some external force. 
This fact is proved by the experience of every person, for we 
see the same objects lying in the same positions all our lives. 
Now, it is just as true, that inert matter has no power to 
bring itself to rest, when once put in motion, as it is that it 
cannot put itself in motion when at rest, for having no life, 
it is perfectly passive, both to motion and rest, and therefore 
either state depends entirely upon circumstances. 

24. Common experience proving that matter does not put 
itself in motion, we might be led to believe, that rest is the 
natural state of all inert bodies, but a few considerations will 
show, that motion is as much the natural state of matter as 
rest, and that either state depends on the resistance, or im¬ 
pulse, of external causes. 

25. If a cannon ball be rolled upon the ground, it will soon 
cease to move, because the ground is rough, and presents 
impediments to its motion; but if it be rolled on the ice, its 
motion will continue much longer, because there are fewer 
impediments, and consequently, the same force of impulse 
will carry it much farther. We see from this, that with the 
same impulse, the distance to which the ball will move must 
depend on the impediments it meets with, or the resistance 
it has to overcome. But suppose that the ball and ice were 
both so smooth as to remove as much as possible the resis¬ 
tance caused by friction, then it is obvious that the ball would 
continue to move longer, and go to a greater distance. Next 
suppose we avoid the friction of the ice, and throw the ball 
through the air, it would then continue in motion still longer 
with the same force of projection, because the air alone pre¬ 
sents less impediment than the air and ice, and there is now 
nothing to oppose its constant motion, except the resistance 
of the air, and its own weight, or gravity. 

26. If the air be exhausted, or pumped out of a vessel by 
means of an air pump, and a common top, with a small, hard 
point, be set in motion in it, the top will continue to spin for 
hours, because the air does not resist its motion. A pendu¬ 
lum, set in motion, in an exhausted vessel, will continue to 
swing, without the help of clock work, for a whole day, 


Is rest or motion the natural state of matter ? Why does the ball roll farthe* 
on the ice than on the ground ? What does this prove? Why, with the sam» 
force of projection, will a ball move farther through the air than on the ice ? 

2 



PROPERTIES OF BODIES, 


14 

because there is nothing to resist its perpetual motion but 
the small friction at the point where it is suspended, and 
gravity. 

27. We see, then, that it is the resistance of the air, of 
friction, and of gravity, which causes bodies once in motion 
to cease moving, or come to rest, and that dead matter, of 
itself, is equally incapable of causing its own motion, or its 
own rest. 

28. We have perpetual examples of the truth of this doc¬ 
trine, in the moon, and other planets. These vast bodies 
move through spaces which are void of the obstacles of air 
and friction, and their motions are the same that they were 
thousands of years ago, or at the beginning of creation. 

29. Attraction. — By attraction is meant that property or 
quality in the particles of bodies, which makes them tend toward 
each other. 

80. We know that substances are composed of small 
atoms or particles of matter, and that it is a collection of 
these^ united together, that forms all the objects with which 
we are acquainted. Now, when we come to divide, or sep¬ 
arate any substance into parts, we do not find that its parti¬ 
cles have been united or kept together by glue, little nails, or 
any such mechanical means, but that they cling together by 
some power, not obvious to our senses. This power we call 
attraction , but of its nature or cause, we are entirely ignorant. 
Experiment and observation, however, demonstrate, that this 
power pervades all material things, and that under different 
modifications, it not only makes the particles of bodies adhere 
to each other, but is the cause which keeps the planets in 
their orbits as they pass through the heavens. 

31. Attraction has received different names, according to 
the circumstances under which it acts. 

32. The force which keeps the particles* of matter together, 
to form bodies, or masses, is called attraction of cohesion. 
That which inclines different masses towards each other, is 
called attraction of gravitation. That which causes liquids 
to rise in tubes, is called capillary attraction. That which 
forces the particles of substances of different kinds to unite, 


Why will a top spin, or a pendulum swing, longer in an exhausted vessel 
than in the air ? What are the causes which resist the perpetual motion of 
nodies? Where have we an example of continued motion without the exist- 
ance of air and friction? What is meant by attraction? What is known 
about the cause of attraction ? Is attraction common to all kinds of matter, or 
not ? What effect does this power have upon the planets ? Why has attrac¬ 
tion received different names ? 



•PROPERTIES OF BODIES. 


1 5 


is known under the name of chemical attraction. That which 
causes the needle to point constantly towards the poles of 
the earth is magnetic attraction; and that which is excited 
by friction in certain substances, is known by the name of 
electrical attraction. 

33. The following illustrations, it is hoped, will make 
each kind of attraction distinct and obvious to the mind of 
the student. 

34. Attraction op Cohesion acts only at insensible distan¬ 
ces , as when the particles of bodies apparently touch each other. 

35. Take two pieces of lead, of a round form, an inch in 
diameter, and two inches long; flatten one end of each, and 
make through it an eye-hole for a string. Make the other 
ends of each as smooth as possible, by cutting them with a 
sharp knife. If now the smooth surfaces be brought to¬ 
gether, with a slight turning pressure, they will adhere with 
such force that two men can hardly pull them apart by the 
two strings. 

36. In like manner, two pieces of plate glass, when their 
surfaces are cleaned from dust, and they are pressed together, 
will adhere with considerable force. Other smooth substan¬ 
ces present the same phenomena. 

37. This kind of attraction is much stronger in some bodies 
than in others. Thus, it is stronger in the petals than in 
most other substances, and in some of the metals it is stronger 
than in others. In general it is most powerful among the 
particles of solid bodies, weaker among those of liquids, and 
probably entirely wanting among elastic fluids, such as air, 
and the gases. 

38. Thus, a small iron wire will hold a suspended weight 
of many pounds, without having its particles separated ; the 
particles of water are divided by a very small force, while 
those of air are still more easily moved among each other. 
These different properties depend on the force of cohesion 
with which the several particles of these bodies are united. 

39. When the particles of fluids are left to arrange them¬ 
selves according to the laws of attraction, the bodies w'hich 
they compose assume the form of a globe or ball. 

40. Drops of water thrown on an oiled surface, or on wax, 


How many kinds of attraction are there? How does the attraction of cohe¬ 
sion operate ? What is meant by attraction of gravitation ? What by capil¬ 
lary attraction ? What by chemical attraction ? What is that which makes 
the needle point towards the pole? How is electrical attraction excited? 
Give an example of cohesive attraction. In what substances is cohesive at¬ 
traction the strongest ? In what substance is it weakest ? 



(l > properties of bodies.- 

—-globules of mercury,—hail-stones,—a drop of water ad¬ 
hering to the end of the finger,—tears running down the 
cheeks, and dew drops on the leaves of plants, are all exam¬ 
ples of this law of attraction. The manufacture of shot is 
also a striking illustration. The lead is melted and poured 
into a sieve, at the height of about two hundred feet from the 
ground. The stream of lead, immediately after leaving the 
sieve, separates into round globules, which, before they reach 
the ground, are cooled and become solid, and thus are formed 
the shot used by sportsmen. 

41. To account for the globular form in all these cases, 
we have only to consider that the particles of matter are mu¬ 
tually attracted towards a common centre, and in liquids 
being free to move, they arrange themselves accordingly. 

4g. In all figures except the globe, or ball, some of the 
particles must be nearer the centre than others. But in a 
body that is perfectly round, every part of the outside is ex¬ 
actly at the same distance from the centre. 

43. Thus, the corners of a cube, 
or square, are at much greater dis¬ 
tances from the centre than the sides, 
while the circumference of a circle or 
ball is every where at the same dis¬ 
tance from it. This difference is 
shown by fig. 1, and it is quite obvi¬ 
ous, that if the particles of matter are 
equally attracted towards the com¬ 
mon centre, and are free to arrange 
themselves, no other figure could possibly be formed, since 
then every part of the outside is equally attracted. 

44. The sun, earth, moon, and indeed all the heavenly 
bodies, are illustrations of this law, and therefore were pro¬ 
bably in so soft a state when first formed, as to allow their 
particles freely to arrange themselves accordingly. 

45. Attraction of Gravitation. — As the attraction of 
cohesion unites the particles of matter into masses or bodies, so 
the attraction of gravitation tends to force these masses towards 
each other, to form those of still greater dimensions. The 
term gravitation, does not here strictly refer to the weight of 



Why are the particles of fluids more easily separated than those of solids ? 
What form do fluids take, when their particles are left to their own arrange¬ 
ment ? Give examples of this law. How is the globular form which liquids 
assume accounted for ? If the particles of a body are free to move, and are 
equally attracted towards the centre, what must be its figure T Why must the 
figure be a globe T 







PROPERTIES OF BODIES. 


17 


bodies, but to the attraction of the masses of matter towards 
each other, whether downwards, upwards, or horizontally. 

46. The attraction of gravitation is mutual, since all 
bodies not only attract other bodies, but are themselves at¬ 
tracted. 

47. Two cannon balls, when suspended by Fig. 2. 
long cords, so as to hang quite near each oth¬ 
er, are found to exert a mutual attraction, so 
that neither of the cords are exactly perpendicu¬ 
lar, but they approach each other, as in fig. 2. 

48. In the same manner, the heavenly bo¬ 
dies, when they approach each other, are 
drawn out of the line of their paths, or orbits, 
by mutual attraction. 

49. The force of attraction increases in pro¬ 
portion as bodies approach each other, and by 
the same law it must diminish in proportion 
as they recede from each other. 

50. Attraction, in technical language, is in¬ 
versely as the squares of the distances between 
the two bodies. That is, in proportion as the 
square of the distance increases, in the same 
proportion attraction decreases, and so the contrary. Thus, 
if at the distance of 2 feet, the attraction be equal to 4 
pounds, at the distance of 4 feet, it will be only 1 pound; for 
the square of 2 is 4, and the square of 4 is 16, which is 4 
times the square of 2. On the contrary, if the attraction at 
the distance of 6 feet be 3 pounds, at the distance of 2 feet it 
will be 9 times as much, or 27 pounds, because 36, the 
square of 6, is equal to 9 times 4, the square of 2. 

51. The intensity of light is found to increase and dimin¬ 
ish in the same proportion. Thus, if a board a foot square, 
be placed at the distance of one foot from a candle, it will be 
found to hide the light from another board of two feet square, 
at the distance of two feet from the candle. Now a board 
of two feet square is just four times as large as one of one 
foot square, and therefore the light at double the distance 
being spread over 4 times the surface, has only one fourth 
the intensity. 


What great natural bodies are examples of this law? What is meant by 
attraction of gravitation ? Can one body attract another without being itself 
attracted ? How is it proved that bodies attract each other ? By what law. 
or rule, does the force of attraction increase ? Give an example of this rule. 

2 # 






18 

52. The ex¬ 
periment may 
be easily tried, 
or may be rea¬ 
dily understood 
by fig. 3, where 
c rep v esents the 
candle, a the 

small board, and b the large one; b being four times the size 
of a. 

The force of the attraction of gravitation, is in proportion 
to the quantity of matter the attracting body contains. 

Some bodies of the same bulk contain a much greater 
quantity of matter than others; thus a piece of lead contains 
about twelve times as much matter as a piece of cork of the 
same dimensions, and therefore a piece of lead of any given 
size, and a piece of cork twelve times as large, will attract 
each other equally. 

53. Capillary Attraction.— The force by which small 
tubes , or porous substances , raise liquids above their levels, is 
called capillary attraction. 

If a small glass tube be placed in water, the water on the 
inside will be raised above the level of that on the outside of 
the tube. The cause of this seems to be nothing more than 
the ordinary attraction of the particles of matter for each 
other. The sides of a small orifice are so near each other, 
as to attract the particles of the fluid on their opposite sides, 
and as all attraction is strongest in the direction of the great¬ 
est quantity of matter, the water is raised upwards, or in the 
direction of the length of the tube. On the outside of the 
tube, the opposite surfaces, it is obvious, cannot act on the 
same column of water, and therefore the influence of attrac¬ 
tion is here hardly perceptible in raising the fluid. This 
seems to be the reason why the fluid rises higher on the in¬ 
side than on the outside of the tube. 

54. Diminution of Density .—In addition to attraction, as 
a cause by which water is sustained in capillary tubes, that 
of the rapid diminution of the density of the fluid at the sur- 


PROPERTIES OF BODIES. 

Fig. 3. 



How is it shown that the intensity of light increases and diminishes in the 
same proportion as the attraction of matter? Do bodies attract in proportion 
to bulk, or quantity of matter? What would be the difference of attraction 
between a cubic inch of lead, and a cubic inch of cork ? Why would there be 
so much difference ? What is meant by capillary attraction ? How is this 
kind of attraction illustrated with a glass tube ? 














PROPERTIES OF BODIES. 


19 


face, has been suggested. This circumstance, though it has 
been entirely neglected by former inquirers, is not only es¬ 
sential to the true investigation of the effects of capillary ac¬ 
tion, but it has been demonstrated, that if there was no loss 
of density at the surface of the liquid, it would always re¬ 
main plane and horizontal in the tube. 

55. It is well known that mercury in a small vertical tube 
is depressed around the sides next the glass, but rises in the 
centre, forming the section of a ball. This is owing to the 
strong attraction the particles of this metal have for each 
other, while they appear to have none for the glass. This 
attraction is beautifully shown by the little bright globules 
which mercury forms on being thrown on a smooth surface. 

56. A great variety of porous substances are capable of 
this kind of attraction. If a piece of sponge or a lump of 
sugar be placed, so that its lowest corner touches the water, 
the fluid will rise up and wet the whole mass. In the same 
manner, the wick of a lamp will carry up the oil to supply 
the flame, though 4he flame is several inches above the level 
of the oil. If the end of a towel happens to be left in a basin 
of water, it will empty the basin of its contents. And on the 
same principle, when a dry wedge of wood is driven into the 
crevice of a rock, and afterwards moistened with water, as 
when the rain falls upon it, it will absorb the water, swell, 
and sometimes split the rock. In Germany, mill-stone quar¬ 
ries are worked in this manner. 

57. Chemical Attraction takes place between the parti¬ 
cles of substances of different kinds y and unites them into one 
compound. 

58. This species of attraction takes place only between 
the particles of certain substances, and is not, therefore, a 
universal property. It is also known under the name of 
chemical affinity , because it is said that the particles of sub¬ 
stances having an affinity between them, will unite, while 
those having no affinity for each other do not readily enter 
into union. 

59. There seems, indeed, in this respect, to be very singu- 


Why does the water rise higher in the tube than it does on the outside ? 
Give some common illustrations of this principle? Why is mercury in a tube 
convex on the surface ? What is said of diminutive density in accounting for 
capillary attraction ? Why does mercury form a section of a ball in a glass 
tube ? What is the effect of chemical attraction ? By what other name is 
this kind of attraction known? What effect is produced when marble and sul¬ 
phuric acid are brought together? What is the effect when glass and this acid 
are brought together ? What is the reason of this difference ? 



PROPERTIES OF BODIES. 


20 

lar preferences, and dislikes, existing among the particles of 
matter. Thus, if a piece of marble be thrown into sulphuric 
acid, their particles will unite with great rapidity and com¬ 
motion, and there will result a compound differing in all re¬ 
spects from the acid or the marble. But if a piece of glass, 
quartz, gold, or silver, be thrown into this acid, no change is 
produced on either, because their particles have no affinity. 

Sulphur and quicksilver, when heated together, will form 
a beautiful red compound, known under the name of vermil¬ 
ion, , and which has none of the qualities of sulphur or quick¬ 
silver. 

60. Oil and water have no affinity for each other, but pot¬ 
ash has an attraction for both, and therefore oil and water 
will unite when potash is mixed with them. In this man¬ 
ner, the well known article called soap is formed. But the 
potash has a stronger attraction for an acid than it has for 
either the oil or the water ; and therefore when soap is mix¬ 
ed with an acid, the potash leaves the oil, and unites with 
the acid, thus destroying the old compound, and at the same 
instant forming a new one. The same happens when soap 
is dissolved in any water containing an acid, as the water of 
the sea, and of certain wells. The potash forsakes the oil, 
and unites with the acid, thus leaving the oil to rise to the 
surface of the water. Such waters are called hard , and will 
not wash, because the acid renders the potash a neutral 
substance. 

61 . Magnetic Attraction.— There is a certain ore of iron, 
a piece of which, being suspended by a thread, will always 
turn one of its sides to the north. This is called the load¬ 
stone or natural magnet, and when it is brought near a piece 
of iron, or steel, a mutual attraction takes place, and under 
certain circumstances, the two bodies will come together and 
adhere to each other. This is called Magnetic Attraction. 
When a piece of steel or iron is rubbed with a magnet, the 
same virtue is communicated to the steel, and it will attract 
other pieces of steel, and if suspended by a string, one of its 
ends will constantly point towards the north, while the other, 
of course, points towards the south. This is called an arti¬ 
ficial magnet. The magnetic needle is a piece of steel, first 


How may oil and water be made to unite ? What is the composition thus 
formed called ? How does an acid destroy this compound ? What is the rea¬ 
son that hard water will not wash ? What is a natural magnet ? What is 
meant by magnetic attraction ? What is an artificial magnet ? What is a mag¬ 
netic needle 7 What is its use 7 



PROPERTIES OF BODIES. 


21 


touched with the loadstone, and then suspended, so as to turn 
easily on a point. By means of this instrument, the mariner 
guides his ship through the pathless ocean. See Magnetism. 

62. Electrical Attraction. —When a piece of glass, oi 
sealing-wax, is rubbed with the dry hand, or a piece of cloth, 
and then held towards any light substance, such as hair, or 
thread, the light body will be attracted by it, and will adhere 
for a moment to the glass or wax. The influence which 
thus moves the light body is called Electrical Attraction. 
When the light body has adhered to the surface of the glass 
for a moment, it is again thrown oflf, or repelled, and this is 
called Electrical Repulsion. See Electricity. 

63. We have thus described and illustrated all the univer¬ 
sal or inherent properties of bodies, and have also noticed the 
several kinds of attraction which are peculiar, namely, 
Chemical, Magnetic, and Electrical There are still several 
properties to be mentioned. Some of them belong to certain 
bodies in a peculiar degree, while other bodies possess them 
but slightly. Others belong exclusively to certain substan¬ 
ces, and not at all to others. These properties are as fol¬ 
lows. 

64. Density. —This property relates to the compactness of 
bodies , or the number of particles which a body contains with¬ 
in a given bulk. It is closeness of texture. Bodies which 
are most dense, are those which contain the least number of 
pores. Hence the density of the metals is much greater 
than the density of wood. Two bodies being of equal bulk, 
that which weighs most, is most dense. Some of the metals 
may have this quality increased by hammering, by which 
their pores are filled up and their particles are brought near¬ 
er to each other. The density of air is increased by forcing 
more into a close vessel than it naturally contained. 

65. Rarity. — This is the quality opposite to density , and 
means that the substance to which , it is applied is porous and 
light. Thus air, water, and ether, are rare substances, while 
gold, lead, and platina, are dense bodies. 

66. Hardness. —This property is not in proportion , as 
might be expected , to the density of the substance , but to the 
force with which the particles of a body cohere , or keep their 


What is meant by electrical attraction? What is electrical repulsion? 
What is density ? What bodies are most dense ? How may this quality be 
increased in the metals ? What is rarity ? What are rare bodies ? What 
are dense bodies ? How does hardness differ from density ? Why will glass 
scratch gold or platina ? 



22 


PROPERTIES OF BODIES. 


places . Glass, for instance, will scratch gold or platina, 
though these metals are much more dense than glass. It is 
probable, therefore, that these metals contain the greatest 
number of particles, but that those of the glass are more firm¬ 
ly fixed in their places. 

Some of the metals can be made harder soft at pleasure. 
Thus steel when heated, and then suddenly cooled, becomes 
harder than glass, while if allowed to cool slowly, it is soft 
and flexible. 

67. Elasticity is that property in bodies by which, after 
being forcibly compressed or bent , they regain their original 
state when the force is removed. 

Some substances are highly elastic, while others want this 
property entirely. The separation of two bodies after im¬ 
pact, or striking together, is a proof that one or both are 
elastic. In general, most hard and dense bodies possess 
this quality in greater or less degree. Ivory, glass, marble, 
flint, and ice, are elastic solids. An ivory ball, dropped upon 
a marble slab, will bound nearly to the height from which it 
fell, and no mark will be left on either. India rubber is ex¬ 
ceedingly elastic, and on being thrown forcibly against a 
hard body, will bound to an amazing distance. 

Putty, dough, and wet clay, are examples of the entire 
want of elasticity, and if either of these be thrown against 
an impediment, they will be flattened, stick to the place 
they touch, and never, like elastic bodies, regain their former 
shapes. 

Among fluids, water, oil, and in general all such substan¬ 
ces as are denominated liquids, are nearly inelastic, while 
air and the gaseous fluids, are the most elastic of all 
bodies. 

68. Brittleness is the property which renders substances 
easily broken , or separated into irregular fragments. This 
property belongs chiefly to hard bodies. 

It does not appear that brittleness is entirely opposed to 
elasticity, since in many substances, both these properties are 
united. Glass is the standard, or type of brittleness, and yet 
a ball, or fine threads of this substance, are highly elastic, 
as may be seen by the bounding of the one, and the spring- 


What metal can be made hard or soft at pleasure ? What is meant by elas¬ 
ticity ? How is it known that bodies possess this property ? Mention sever¬ 
al elastic solids. Give examples of inelastic solids. Do liquids possess this 
property ? What are the most elastic of all substances ? What is brittleness ? 
Are brittleness and elasticity ever found in the same substance? Give ex 
amples. 



PROPERTIES OF BODIES. 


23 


itig of the other. Brittleness often results from the treatment 
to which substances are submitted. Iron, steel, brass, and 
copper, become brittle when heated and suddenly cooled; 
but if cooled slowly, they are riot easily broken. 

69. Malleability. —Capability of being drawn under the 
hammer , or rolling press. This property belongs to some of 
the metals, but not to all, and is of vast importance to the 
arts and conveniences of life. 

The malleable metals are, gold, silver, iron, copper, and 
some others. Antimony, bismuth, and cobalt, are brittle met¬ 
als. Brittleness is therefore the opposite of malleability. 

Gold is the most malleable of all substances. It may be 
drawn under the hammer so thin that light may be seen 
through it. Copper and silver are also exceedingly malle¬ 
able. 

70. Ductility, is that property in substances which renders 
them susceptible of being drawn into wire. 

We should expect that the most malleable metals would 
also be the most ductile ; but experiment proves that this is 
not the case. Thus, tin and lead may be drawn into thin 
leaves, but cannot be drawn into small wire. Gold is the 
most malleable of all the metals, but platina is the most duc¬ 
tile. Dr. Wollaston drew platina into threads not much lar¬ 
ger than a spider’s web. 

71. Tenacity, in common language called toughness, refers 
to the force of cohesion among the particles of bodies. Tena¬ 
cious bodies are not easily pulled apart. There is a remarka¬ 
ble difference in the tenacity of different substances. Some 
possess this property in a surprising degree, while others are 
torn asunder by the smallest force. 

72. Tenacity of Wood. —The following is a tabular view 
of the absolute cohesion of the principal kinds of timber em¬ 
ployed in the arts, and in building, showing the weight 
which would rend a rod an inch square, and also the length 
of the rod, which if suspended, would be tom asunder by its 
own weight. 

73. It appears by experiment, that the following is the 
average tenacity of the kinds of wood named, but it is found 
that there is much difference in the strength of the same spe¬ 
cies of wood and even of the different parts of the same tree. 


How are iron, steel, and brass, made brittle? What does malleability 
mean ? What metals are malleable, and what are brittle ? Which is the 
most malleable metal? What is meant by ductility ? Are the most mallea¬ 
ble metals the most ductile ? What is meant by tenacity 7 From what does 
this property arise 7 



24 


PROPERTIES OF BODIES. 


74. The first line refers to the weight, and the other to 


the length. 

Pound*. Feet. 

Teak, .... 12,915 .... 36,049 

Oak, .... 11,880 .... 32,900 

Sycamore, . . 9,630 .... 35,800 

Beech, . . . 12,225 .... 38,940 

Ash, .... 14,130 ... . 39,050 

Elm, .... 9,540 .... 40,500 

Larch, .... 12,240 .... 42,160 


75. Tenacity of the Metals. —The metals differ much 
more widely in their tenacity than the woods. Accord¬ 
ing to the experiments of Mr. Rennie, the cohesive power of 
the several metals named below, each an inch square, is 
equal to the number of pounds marked in the table, while the 
feet indicate the length required to separate each metal by 
its own weight. 

Pounds. Feet. 


39,455 
19,740 
6,110 
5,180 
5,093 
1,496 
348 

to measure, though 


Cast steel, . . 134,256 

Malleable iron, . 72,064 

Cast iron, . . . 19,096 

Yellow brass, . . 17,958 

Cast copper, . . 19,072 

Cast tin, . . . 4,736 

Cast lead, . . . 1,824 

The cohesion of fluids, it is difficult 
some indication of this property is derived by the different 
sizes of the drops of each On a plane surface. 

76. Recapitulation. —The common, or essential proper¬ 
ties of bodies, are, Impenetrability, Extension, Figure, Divisi¬ 
bility, Inertia, and Attraction. Attraction is of several kinds, 
namely, Attraction of cohesion, Attraction of gravitation, 
Capillary attraction, Chemical attraction, Magnetic attrac¬ 
tion, and Electrical attraction. 

77. The peculiar properties of bodies are, Density, Rarity, 
Hardness, Elasticity, Brittleness, Malleability, Ductility, and 
Tenacity. 


FORCE OF GRAVITY. 


78. The force hy which bodies are drawn towards each other 
in the mass, and by which they descend towards the earth when 
suspended or let fall from a height, is called the force of 
gravity. 

What metals are most tenacious? What metals are least tenacious? 
What are the essential properties of bodies ? How many kinds of attraction 
v ere ? What are the peculiar properties of bodies ? What is gravity? 












> 


bKAVITV. 


25 


79.<The attraction which the earth exerts on all bodies , 
neai its surface, is called terrestrial gravity , and the force 
with which any substance is drawn downwards, is called its 
weight. 

80.,/All falling bodies tend downwards towards the centre 
of the earth, in a straight line from the point where they are 
let fall. If then a body be let fall in any part of the world, 
the line of its direction will be perpendicular to the earth’s 
surface. It follows, therefore, that two falling bodies, on op¬ 
posite parts of the earth, mutually fall towards each other. 

81. Suppose a cannon ball to be disengaged from a height 
opposite to us, on the other side of the earth, its motion in re¬ 
spect to us would be upward, while the downward motion 
from where we stand, would be upward in respect to those 
who stand opposite to us, on .he other side of the earths 

82. [In like manner if the falling body be a quarter, instead 

of half the distance round the earth from us, its line of direc- " 
tion would be directly across, or at right-angles with the line 
already supposed./ 

83. /This will be rea- Fig. 4. 

dily understood by fig. 4, 
where the circle is sup¬ 
posed to be the circum¬ 
ference of the earth, <z, 
the ball falling towards 
its upper surface, where 
we stand ; 6, a ball fall¬ 
ing towards the oppo¬ 
site side of the earth, 
but ascending in respect 
to us ; and d, a ball de¬ 
scending at the distance 
of a quarter of the cir¬ 
cle, from the other two, 
and crossing the line of 
their direction at right- 
angles. ) 

84. It will be obvious, therefore, that what we call up and 
down are merely relative terms, and that what is down in re¬ 
spect to us, is up in respect to those who live on the oppo- 



What is terrestrial gravity ? To what point in the earth do falling bodies 
tend ? In what direction will two falling bodies from opposite parts of the 
earth tend, in respect to each other? In what direction will one from half 
way between them meet their line ? How is this shown by the figure ? Are 
the terms up and down relative, or positive, in their meaning ? 

8 





26 


gravity. 


silo side of the earth, and so the contrary Consequently, 
down every where means towards the centre of the earth, 
and up, from the centre of the earth, because all bodies de¬ 
scend towards the earth’s centre, from whatever part they are 
let fall. This will be apparent when we consider, that as 
the earth turns over every 24 hours, we are carried with it 
through the points a, d, and h, fig. 4 ; and therefore,(if a ball 
is supposed to fall from the point a, say at 12 o clock, and 
the same ball to fall again from the same point above the 
earth, at 6 o’clock, the two lines of direction will be at nght- 
angles, as represented in the figure, for that part of the earth 
which was under a at 12 o’clock, will be under d at 6 o clock, 
the earth having in that time performed one quarter of its 
daily revolution) At 12 o’clock at night, if the ball be sup¬ 
posed to fall again, its line f direction will be at right- 
angles with that of its last descent, and consequently it will 
ascend in respect to the point on which it fell 12 hours be¬ 
fore, because the earth would have then gone through one 
half her daily rotation, and the point a would be at b.) 

JThe velocity or rapidity of every falling body, is uniform- 
' ^accelerated, or increased, in its approach towards the earth, 
from whatever height it falls. 

85. If a rock is rolled from a steep mountain, its motion is 
at first slow and gentle, but as it proceeds downwards, it 
moves with perpetually increased velocity^ seeming to gath¬ 
er fresh speed every moment, until its force is such that eve¬ 
ry obstacle is overcome ; trees and rocks are beat from its 
path, and its motion doers not cease until it has rolled to a 
great distance on the pla n. 

VELOCITY OF FALLING BODIES. 

86. The same princij le of increased velocity as bodies de¬ 
scend from a height, is curiously illustrated by pouring mo¬ 
lasses or thick syrup fnm an elevation to the ground. The 
bulky stream, of perh; ips two inches in diameter, where it 
leaves the vessel, as it descends, is reduced to the size of a 
straw, or knitting needle ; but what it wants in bulk is made 
up in velocity, for the small stream at the ground will fill a 
vessel just as soon as the large one at the outlet. 

What is understood by down in any part of the earth?. Suppose a ball be let 
fall at 12 and then at 6 q’clock, in what direction would the lines of their de¬ 
scent meet each other ? Suppose two balls to descend from opposite sides ot 
the earth, what would be their direction in respect to each other?; What is 
said concerning the motions of falling bodies ? How is this increased velocity 
illustrated? 



GRAVITY. 


2 7 


87. For the same reason, a man may leap from a chair 
without danger, but if he jumps from the house top, his ve- / 
locity becomes so much increased, before he reaches the 
ground, as to endanger his life by the blow. 

It is found by experiment, that the motion of a falling body 
is increased, or accelerated, in regular mathematical propor¬ 
tions. 

88. These increased proportions do not depend on the in¬ 
creased weight of the body, because it approaches nearer the 
centre of the earth, but on the constant operation of the force 
of gravity, which perpetually gives new impulses to the fall¬ 
ing body, and increases its velocity. 

89. It has been ascertained by experiment, that a body, 
falling freely, and without resistance, passes through a space 
of 16 feet and 1 inch during the first second of time. Leav 
ing out the inch, which is not necessary for our present pur¬ 
pose, the ratio of descent is as follows. 

90. If the height through which a body falls in one sec¬ 
ond of time be known, the height which it falls in any pro¬ 
posed time may be computed. For since the height is pro¬ 
portional to the square of the time, the height through which 
it will fall in two seconds will be four times that which it falls 
through in one second. In three seconds it will fall through 
nine times that space ; in four seconds sixteen times that of 
the first second ; in five seconds, twenty-five times, and so on 
in this proportion, ) 

91. The following, therefore, is a general rule to find the 
height through which a body will fall in any given time. 

92. Rule—vReduce the given time to seconds; take the 
square of the number of seconds in the time , and multiply the 
height through which the body falls in one second by that num¬ 
ber , and the result will be the height sought. ' 

93. The following table exhibits the height and corres¬ 
ponding times as far as 10 seconds. 


Time 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Height 

1 

4 

9 

16 

25 

36 

49 

64 

81 

100 


94. Each unit in the upper row expresses a second of time, 
and each number in the second row expresses the height 
thi > i gh which a body falls freely in a second. 


Why is there any more danger in jumping from the house top than from 
a chair? What number of feet does a falling body pass through in the first 
second ? . If a body fall from a certain height in two seconds, what proportion 
to this will it fall in four seconds ?- What is the rule by which the height from 
which a body falls may be found ? 














28 


.*>'■ ity 

GRAVITY. 


. r~vi' ' 

t & 6 



95. Now, as the body falls at the rate of 16 feet during 
the first second, this number, according to the rule, multipli¬ 
ed by the square of the time, that is, by the numbers express¬ 
ed in the second line, will show the actual distance through 
which the body falls. 

96. Thus we have for the first second 16 feet; for the end 
of the second, 4 X 16=64 feet; third, 9 X 16= 144 ; fourth, 
16x16=256; fifth, 25x16=400; sixth, 36x 16=576; 
seventh, 49 x 16 = 784 ; and for the 10 seconds 1600 feet. 

97. If, on dropping a stone from a precipice, or into a well, 
we count the seconds from the instant of letting it fall until 
we hear it strike, we may readily estimate the height of the 
precipice, or the depth of the well. Thus suppose it is 5 
seconds in falling, then we only have to square the seconds, 
and multiply this by the distance the body falls in one sec¬ 
ond. We have then 5x5=25, the square, which 25 x 16 
=400 feet, the depth of the well. 

98. Thus it appears, that to ascertain the velocity with 
which a body falls in any given time, we must know how 
many feet it fell during the first second: the velocity ac¬ 
quired in one second, and the space fallen through during 
that time, being the fundamental elements of the whole cal¬ 
culation, and all that are necessary for the computation of 
the various circumstances of falling bodies. 

99. The difficulty of calculating exactly the velocity of 
a falling body from an actual measurement of its height, and 
the time which it takes to reach the ground, is so great, that 
no accurate computation could be made from such an expe¬ 
riment. 

100. Atwood's Machine. —This difficulty has, however, 
been overcome by a curious piece of machinery, invented for 
this purpose by Mr. Atwood. 

101. This machine consists of two upright posts of wood, 
fig. 5, with cross pieces, as shown in the figure. The 
weights a and h , are of the same size, and made to balance 
each other very exactly, and are connected by the thread 
which passes over the wheel c; f, is a ring through which 
the weight a passes, and g is a stage on which the 
weight rests in its descent. The ring and stage both slide 
up and down, and are fixed at pleasure by thumb screws. 
The post h is a graduated scale, and the pendulum k , is kept 

How many feet will a body fall in nine seconds ? Is the velocity of a fall 
ing body calculated from actual measurement, or by a machine ? Describe 
the operation of Mr. Atwood’s machine for estimating the velocities of fall¬ 
ing bodies. 




GRAVITY 


29 


in motion by clock-work; 
/, is a small bar of metal, 
weighing a quarter of an 
ounce, and longer than the 
diameter of the ring /. 

102. When the machine 
is to be used, the weight a is 
drawn up to the top of the 
scale, and the ring and stage 
are placed a certain num¬ 
ber of inches from each other. 
The small bar l, is then pla¬ 
ced across the weight a, by 
means of which it is made 
slowly to descend. When it 
has descended to the ring, 
the small weight Z, is taken 
off by the ring, and thus the 
two weights are left equal to 
each other. Now it must be 
observed, that the motion, 
and descent of the weight a, 
is entirely owing to the gravi- 
tatingforce of the weight Z, 
until it arrives at the ring/, 
when the action of gravity 
is suspended, and the large 
weight continues to move 
downwards to the stage, in 
consequence of the velocity 
that time.} 


Fig. 5. 



it had acquired previously to 


103. To comprehend the accuracy of this machine, it must 
be understood that the velocities of gravitating bodies are 
supposed to be equal, whether they are large or small, this 
being the case when no calculation is made for the resistance 
of the air. Consequently, the weight of a quarter of an 
ounce placed on the large weight a, is a representative of all 
other solid descending bodies. The slowness of its descent, 
when compared with freely gravitating bodies, is only a con¬ 
venience by which its motion can be accurately measured, 


After the small weight is taken off by the ring, why does the large weight 
continue to descend ? Does this machine show the actual velocity of a falling 
\iody, or only its increase ? 

3 * 














30 


GRAVITY. 


for it is the increase of velocity which the machine is designed 
to ascertain, and not the actual velocity of falling bodies. 

104. Now it will be readily comprehended, that in this 
respect, it makes no difference how slowly a body falls, pro¬ 
vided it follows the same laws as other descending bodies , and 
it has already beeen stated, that all estimates on this subject 
are made from the known distance a body descends during 
the first second of time. 

105. It follows, therefore, that if it can be ascertained ex¬ 
actly, how much faster a body falls during the third, fourth, 
or fifth second, than it did during the first second, by know¬ 
ing how far it fell during the first second, we should be able 
to estimate the distance it would fall during all succeeding 
seconds. 

106. If, then, by means of a pendulum beating seconds, 
the weight a should be found to descend a certain number of 
inches during the first second, and another certain number 
during the next second, and so on, the ratio of increased de¬ 
scent would be precisely ascertained, and could be easily 
applied to the falling of other bodies 5 and this is the use to 
which this instrument is applied. 

107. By this machine it can also be ascertained how 
much the actual velocity of a falling body depends on the 
force of gravity, and how much on acquired velocity, for 
the force of gravity gives motion to the descending weight 
only until it arrives at the ring, after which the motion is 
continued by the velocity it had before acquired. 

108. From experiments accurately made with this ma¬ 
chine, it has been fully established, that if the time of a fall- 
irg body be divided into equal parts, say into seconds, the 
spaces through which it falls in each second, taken sepa¬ 
rately, will be as the odd numbers, 1 , 3 , 5 , 7 , 9 , and so on, 
as already stated. To make this plain, suppose the times 
occupied by the falling body to be 1 , 2 , 3 , and 4 seconds; 
then the spaces fallen through will be as the squares of these 
seconds, or times, viz. 1, 4, 9, and 16, the square of 1 being 
1, the square of 2 being 4, the square of 3, 9, and so on. 
The distance fallen through, therefore, during the second 
second, may be found, by taking 1 , the distance correspond¬ 
ing to one second, from 4, the distance corresponding to 2 


How does Mr. Atwood’s machine show how much the celerity of a body de 
pends upon gravity, and how much on acquired velocity ? ^Suppose the times 
of a falling body are as the numbers 1, 2, 3, 4, what will be the numbers repre¬ 
senting the spaces through which it falls ? Suppose a body falls 16 feet the 
first second, how far will it fall the third second T 





GRAVITY. 


31 

seconds, and is therefore 3. For the third second, take 4 
from 9, and therefore the distance will be 5. For the fourth 
second, take 9 from 16, and the distance will be 7, and so 
on. During the first second, then, the body falls a certain 
distance; during the next second, it falls three times that 
distance; during the third, five times the distance; during 
the fourth, seven times that distance, and so continually in 
that proportion. ; 

109. (It will be readily conceived, that solid bodies falling 
from great heights, must ultimately acquire an amazing ve¬ 
locity by this proportion of increase. (An ounce ball of lead, 
let fall from a certain height towardsNhe earth, would thus 
acquire a force ten or twenty times as great as when shot 
out of a rifle/ |By actual calculation, it has been found that 
were the moon to lose her projectile force, which counter¬ 
balances the earth’s attraction, she would fall to the earth 
in four days and twenty hours, a distance of 240,000 miles. 
And were the earth’s projectile force destroyed, it would fall 
to the sun in sixty-four days and ten hours, a distance of 
95,000,000 of miles/* 

110. Every one knows by his own experience the differ¬ 
ent effects of the same body falling from a great or a small 
height. A boy will toss up his leaden bullet and catch it 
with his hand, but he soon learns, by its painful effects, not to 
throw it too high. The effects of hail-stones on window glass, 
animals, and vegetation, are often surprising, and some¬ 
times calamitous illustrations of the velocity of falling bodies. 

111. It has been already stated, that the velocities of solid 
bodies falling from a given height, towards the earth, are 
equal, or in other words, that an ounce ball of lead will de¬ 
scend in the same time as a pound ball of lead. 

112. This is true in theory, but there is a slight difference 
in this respect in favor of the velocity of the larger body, 
owing to the resistance of the atmosphere. We, however, 
shall at present consider all solids, of whatever size, as de¬ 
scending through the same spaces in the same times, this 
being exactly true when they pass without resistance.. 

113. ; To comprehend the reason of this, we have only to 
consider, that the attraction of gravitation in acting on a 


Would it be possible for a rifle ball to acquire a greater force by falling, than 
if shot from a rifle ? g How long would it take the moon to come to the earth 
according to the law of increased velocity ? How long would it take the earth 
to fall to the sun ? , What familiar illustrations are given of the force acquired 
by the velocity of falling bodies ? Will a small and large body fall through the 
game space in the same time ? 



GRAVrn 


32 

mass of matter acts on every particle it contains; and thus 
every particle is drawn down equally and with the same 
force, The effect of gravity, therefore, is in exact proportion 
to the quantity of matter the mass contains, and not in pro- 
portion to its bulk. A ball of lead of a foot in diameter, and 
one of wood of the same diameter, are obviously of the same 
bulk; but the lead will contain twelve particles of matter 
where the wood contains one, and consequently will be at¬ 
tracted with twelve times the force, and therefore will weigh 
twelve times as much. 

114. Attraction proportionable to the quantity of matter.-— 
If, then, bodies attract each other in proportion to the quanti¬ 
ties of matter they contain, it follows that if a mass of the 
earth were doubled, the weights of all bodies on its surface 
would also be doubled; and if its quantity of matter were 
tripled, all bodies would weigh three times as much as they 
do at present, 

115. It follows, also, that two attracting bodies, when free 
to move, must approach each other mutually. If the two 
bodies contain like quantities of matter, their approach will 
be equally rapid, and they will move equal distances towards 
each other. But if the one be small and the other large, the 
small one will approach the other with the rapidity propor¬ 
tioned to the less quantity of matter it contains./ 

116. It is easy to conceive, that if a man in one boat pulls 
at a rope attached to another boat, the two boats, if of the 
same size, will move towards each other at the same rate; 
but if the one be large and the other small, the rapidity with 
which each moves will be in proportion to its size, the large 
one moving with as much less velocity as its size is greater. 

117. A man in a boat pulling a rope attached to a ship, 
seems only to move the boat, but that he really moves the 
ship is certain, when it is considered, that a thousand boats 
pulling in the same manner would make the ship meet them 
half way/ 

118. It appears, therefore, that an equal force acting on 
bodies containing different quantities of matter, move them 
with different velocities, and that these velocities are in an 
inverse proportion to their quantities of matter. 

119. In respect to equal forces , it is obvious that in the 

On what parts of a mass of matter does the force of gravity act ? Is the effect 
of gravity in proportion to bulk, or quantity of matter? Were the mass of the 
earth doubled, how much more should we weigh than ive do now ? Suppose 
one body moving towards another, three times as large, by the force of gravity, 
what would be their proportional velocities ? How is this illustrated ? 



GRAVITY. 


33 


case of the ship and single boat, they were moved towards 
each other by the same force, that is, the force of a man 
pulling by a rope. The same principle holds in respect to 
attraction, for all bodies attract each other equally, according 
to the quantities of matter they contain, and since all attrac¬ 
tion is mutual, no body attracts another with a greater force 
than that by which it is attracted. 

120. ^uppose a body to be placed at a distance from the 
earth, weighing two hundred pounds ; the earth would then 
attract the body with a force equal to two hundred pounds, 

/_and the body would attract the earth with an equal force, 
otherwise their attraction would not be equal and mutual. 
Another body weighing ten pounds, would be attracted with 
a force equal to ten pounds, and so of all bodies according to 
the quantity of matter they contain; each body being at¬ 
tracted by the earth with a force equal to its own weight, 
and attracting the earth with an equal force. 

121. If, for example,(Jwo boats be connected by a rope, j, 
and a man in one of them pulls with a force equal to 100 
pounds, it is plain that the force on each vessel would be 100 
pounds. For, if the rope were thrown over a pulley, and a 
man were to pull at one end with a force of 100 pounds, it is 
plain it would take 100 pounds at the other end to balance. 

122. Attracting bodies approach each other .is inferred 
from the above principles, that all attracting bodies which are 
free to move, mutually approach each other, and therefore 
that the earth moves towards every body which is raised from 
its surface, with a velocity and to a distance proportional to 
the quantity of matter thus elevated from its surface. But 
the velocity of the earth being as many times less than that 
of the falling body as its mass is greater, it follows that its 
motion is not perceptible to us. / 

123. The following calculation will show what an im¬ 
mense mass of matter it would take, to disturb the earth’s 
gravity in a perceptible manner. 

124. If a ball of earth equal in diameter to the tenth part 
of a mile, were placed at the distance of the tenth part of a 


Does a large body attract a small one with any more force than it is attract¬ 
ed 1 Suppose a body weighing 200 pounds to be placed at a distance from the 
earth, with how much force does the earth attract the body? With what force 
does the body attract the earth ? Suppose a man in one boat, pulls with a 
force of 100 pounds at a rope fastened to another boat, what would be the force 
on each boat ? How is this illustrated ? Suppose the body falls towards the 
earth, is the earth set in motion by its attraction ? Why is not the earth’s mo¬ 
tion towards it perceptible 7 



34 


FALLING BODIES. 


I 


mile from the earth’s surface, the attracting powers of the 
two bodies would be in the ratio of about 512 millions of mil¬ 
lions to one. For the earth’s diameter being about 8000 
miles, the tWo bodies would bear to each other about this 
proportion. Consequently, if the tenth part of a mile were 
divided into 512 millions of millions of equal parts, one of 
these parts would be nearly the space through which the 
earth would move towards the falling body. Now, in the 
tenth part of a mile there are about 6400 inches, consequently 
this number must be divided into 512 millions of millions of 
parts, which would give the eighty thousand millionth part 
of an inch through which the earth would move to meet a 
body the tenth part of a mile in diameter, j 


ASCENT OF BODIES. 

125. Having now explained and illustrated the 
influence of gravity on bodies moving downward 
and horizontally, it remains to show how matter 
is influenced by the same power when bodies are 
moved upward, or contrary to the force of gravity. 

What has been stated in respect to the velocity 
of falling bodies is exactly reversed in respect to 
those which are thrown upwards, for as ithe mo¬ 
tion of a falling body is increased by the Action of 
gravity, so is it retarded by the same force when 
thrown upwards. 

A bullet shot upwards, every instant loses a part 
of its velocity, until having arrived at the highest 
point from whence it was thrown, it then returns 
again to the earth. 

The same law that governs a descending body, 
governs an ascending one, only that their motions 
are reversed. 

The same ratio is observed to whatever distance 
the ball is propelled, or as the height to which it is 
thrown may be estimated from the space it passes 
through during the first second, so its returning 
velocity is in a like ratio to the height to which it 
was sent. 

This will be understood by fig. 6. Suppose a 
ball to be propelled from the point a, with a force 
which would carry it to the point b in the first 
second, to c in the next, and to d in the third sec¬ 
ond. It would then remain nearly stationary for 


Fig. 6. 

C- 


b 


Ob 




FALLING BODIES. 


35 


an instant, and in returning would pass through exactly the 
same spaces in the same times, only that its direction would 
be reversed. Thus it will fall from d to c, in the first second, 
to b in the next, and to a in the third. 

Now the force of a moving body is as its velocity and its 
quantity of matter, and hence the same ball will fall with 
exactly the same force that it rises. For instance, a Dali 
shot out of a rifle, with a force sufficient to overcome a cer¬ 
tain impediment, on returning would again overcome the 
same impediment. 

FALL OF LIGHT BODIES. 

126. It has been stated that the earth’s attraction acts 
equally on all bodies, containing equal quantities of matter, 
and that in vacuo, all bodies, whether large or small, descend 
from the same heights in the same time. 

127. There is, however, a great difference in the quanti¬ 
ties of matter which bodies of the same bulk contain, and 
consequently a difference in the resistance which they meet 
with in passing through the air. 

128. Now, the fall of a body containing a large quantity 
of matter in a small bulk, meets with little comparative re¬ 
sistance, while the fall of another, containing the same quan¬ 
tity of matter, but of larger size, meets with more in compa¬ 
rison, for it is easy to see that two bodies of the same size 
meet with exactly the same actual resistance. Thus if we 
let fall a ball of lead, and another of cork, of two inches in 
diameter each, the lead will reach the ground before the cork, 
because, though meeting with the same resistance, the lead 
has the greatest power of overcoming it. 

129. This, however, does not affect the truth of the gen¬ 
eral law already established, that the weights of bodies are as 
the quantities of matter they contain. It only shows that the 
pressure of the atmosphere prevents bulky and porous sub¬ 
stances from falling with the same velocity as those which 
are compact or dense. 

130. Were the atmosphere removed, all bodies, whether 


What distance would a body the tenth part of a mile in diameter, placed at 
the distance of a tenth part of a mile, attract the earth towards it ? What effect 
does the force of gravity have on bodies moving upward? Are upward and 
downward motion governed by the same laws ? Explain fig. 6. What is the 
difference between the upward and returning velocity of the same body ? 

Why will not a sack of feathers and a stone of the same size fall through the 
air in the same time ? Does this affect the truth of the general law, that the 
weights of bodies are as their quantities of matter ? 



MOTION. 


36 

light or heavy, large or small, would descend with the same 
velocity. This fact has been ascertained by experiment in 
the following manner: 

131. The air pump is an instrument, by means of which 
the air can be pumped out of a close vessel, as will be seen 
under the article Pneumatics. Taking this for granted al 
present, the experiment is made in the following manner: 

132. On the plate of the air pump a, Fis> 7> 

place the tall jar b , which is open at the 
bottom, and has a brass cover fitted 
closely to the top. Through the cover 
let a wire pass, air tight, having a 
small cross at the lower end. On each 
side of this cross, place a little stage, 
and so contrive them that by turn¬ 
ing the wire by the handle c, these 
stages shall be upset. On one of the 
stages place a guinea or piece of lead, 
and on the other place a feather. When 
this is arranged, let the air be exhausted 
from the jar by the pump, and then turn 
the handle c, so that the guinea and 
feather may fall from their places, and 
it will be found that they will both strike 
the plate at the same instant. Thus is 
it demonstrated, that were it not for the 
resistance of the atmosphere, a bag of 
feathers and one of guineas would fall 
from a given height with the same ve¬ 
locity and in the same time. 

MOTION. 

133. Motion may be defined , a continued change of place 
with regard to a fixed point. 

134. Without motion there would be no rising or setting 
of the sun—no change of seasons—no fall of rain—no build¬ 
ing of houses, and finally no animal life. Nothing can be 
done without motion, and therefore without it, the whole 
universe would be at rest and dead. 


C 



What would be the effect on the fall of light and heavy bodies, were the at¬ 
mosphere removed ? How is it proved that a feather and a guinea will fall 
through equal spaces in the same time, where there is no resistance ? 

How will you define motion ? What would be the consequence were all 
motion to oeoee ? 








VELOCITY OF MOTION. 


67 

135. In the language of philosophy, the power which puts 
a body in motion, is called force. Thus it is the force of 
gravity that overcomes the inertia of bodies, and draws them 
towards the earth. The force of water and steam gives mo¬ 
tion to machinery, &c. 

136. For the sake of convenience, and accuracy in the 
application of terms, motion is divided into two kinds, viz.: 
absolute and relative. 

137. Absolute motion is a change of place with regard to 
a. fixed point, and is estimated without reference to the mo¬ 
tion of any other body. When a man rides along the street, 
or when a vessel sails through the water, they are both in 
absolute motion. 

138. Relative motion is a change of place in a body, with 
respect to another body, also in motion, and is estimated 
from that other body, exactly as absolute motion is from a 
fixed point. 

139. The absolute velocity of the earth in its orbit from 
west to east, is 68,000 miles in an hour; that of Mars, in 
the same direction, is 55,000 miles per hour. The earth’s 
relative velocity, in this case, is 13,000 miles per hour from 
west to east. That of Mars, comparatively, is 13,000 miles 
from east to west, because the earth leaves Mars that dis¬ 
tance behind her, as she would leave a fixed point. 

140. Rest , in the common meaning of the term, is the 
opposite of motion, but it is obvious that rest is often a rela¬ 
tive term, since an object may be perfectly at rest with re¬ 
spect to some things, and in rapid motion in respect to others. 
Thus, a man sitting on the deck of a steamboat, may move 
at the rate of fifteen miles per hour, with respect to the land, 
and still be at rest with respect to the boat. And so, if an¬ 
other man was running on the deck of the same boat at the 
rate of fifteen miles the hour in a contrary direction, he would 
be stationary in respect to a fixed point, and still be running 
with all his might, with respect to the boat. 

VELOCITY OF MOTION. 

141. Velocity is the rate of motion at which a body moves 
from one place to another. 

142. Velocity is independent of the weight or magnitude 


What is that power called which puts a body in motion l How is motion 
divided ? What is absolute motion ? What is relative motion ? What is the 
earth’s relative velocity in respect to Mars? In what respect is a man in a 

steamboat at rest, and in what respect does he move ? What is vetooity T 

4 



38 




MOMENTUM. 


ol the moving body. Thus a cannon ball and a musket, bull, 
noth flying at the rate of a thousand feet in a second, have 
the same velocities. 

143. Velocity is said to be uniform , when the moving body 
passes over equal spaces in equal times. If a steamboat 
moves at the rate of ten miles every hour, her velocity is 
uniform. The revolution of the earth from west to east is a 
perpetual example of uniform motion. 

144. Velocity is accelerated , when the rate of motion is 
constantly increased, and the moving body passes through 
unequal spaces in equal times. Thus, when a falling body 
moves sixteen feet during the first second, and forty-eight 
feet during the next second, and so on, its velocity is accele¬ 
rated. A body falling from a height freely through the air, 
is the most perfect example of this kind of velocity. 

145. Retarded velocity , is when the rate of motion of the 
body is constantly decreased, and it is made to move slower 
and slower. A ball throyn upwards into the air, has its 
velocity constantly retarded by the attraction of gravitation, 
and consequently, it moves slower every moment. 

FORCE, OR MOMENTUM OF MOVING BODIES. 

146. The velocities of bodies are equal, when they pass 
over equal spaces in the same times; but the force with 
which bodies, moving at the same rate, overcome impedi¬ 
ments, is in proportion to the quantity of matter they contain. 
This power, or force, is called the momentum of the moving 
body 

147. Thus, if two bodies of the same weight move with 
the same velocity, their momenta will be equal. 

148. Two vessels, each of a hundred tons, sailing at the 
rate of six miles an hour, would overcome the same impedi¬ 
ments, or be stopped by the same obstructions. Their mo¬ 
menta would therefore be the same 

149. The force or momentum of a moving body, is in pro¬ 
portion to its quantity of matter, and its velocity. 

150. A large body moving slowly, may have less momen¬ 
tum than a small one moving rapidly. Thus, a bullet shot 


When is velocity uniform ? When is velocity accelerated ? Give illustra¬ 
tions of these two kinds of velocity. What is meant by retarded velocity ? 
Give an example of retarded velocity. What is meant by the momentum of a 
body? When will the momenta of two bodies be equal? Give an example 
When has a small body more momentum than a large one ? 



MOMENTUM. 


39 


out of a gun, moves with much greater force than a stone 
thrown bj the hand. 

151. The momentum of a body is found by multiplying its 
quantity of matter by its velocity per second. Thus, if the 
velocity be 2, and the weight 2, the momentum will be 4. 

If the velocity be 6, and the weight of the body 4, the mo¬ 
mentum will be 24. 

152. If a moving body strikes an impediment, the force 
with which it strikes, and the resistance of the impediment, 
are equal. Thus, if a boy throw his ball against the side of 
the house, with the force of 3, the house resists it with an 
equal force, and the ball rebounds. If he throws it against 
a pane of glass with the same force, the glass having only 
the power of 2 to resist, the ball will go through the glass, 
still retaining one third of its force. 

153. Pile Driver .—This machine consists of a frame 
and pulley, by which a large piece of cast iron, called the 
hammer , is raised to the height of 30 or 40 feet, and then let 
fall on the end of a beam of wood Called a pile , and by which 
it is driven into the ground. When the hammer is large, 
and the height considerable, the force, or momentum, is tre¬ 
mendous, and unless the pile is hooped with iron, will split it 
into fragments. 

154. Now the momentum of a body is proportional to its 
weight and velocity conjointly, and, therefore, to find it, we 
must multiply their two sums together. 

Suppose the hammer, weighing 2000 pounds, is ele¬ 
vated two seconds of time above the head of the pile, then, 
according to the law of falling bodies, already stated, it would 
have fallen 64 feet, this being at that instant the rate of its 
velocity. Then 64 x 2000, being the velocity and quantity 
of matter, gives 64 tons as the momentum. But according 
to the same law, this force is immensely increased by a small 
increase of time, for if we add two seconds of time, the rate 
of velocity will be 256 feet per second, and thus 256 x 2000 
=512,000 pounds, or 256 tons. 

155. Action and re-action equal .—From observations made 
on the effects of bodies striking each other, it is found that ./ 
action and re-action are equal; or, in other words, that force ' 


By what rule is the momentum of a body found ? When a moving body strikes 
an impediment, which receives the greatest shock ? WTiat is a pile driver ? If 
the hammer of this machine weighs 2000 pounds, and falls 2 seconds, what will 
be the momentum ? If the fall be 3 seconds, what is the momentum ? How is 
the momentum of a falling body found? I What is the law of action and re 
action ■ 



40 


MOMENTUM. 


£ 


and resistance are equal. Thus, when a moving* body 
strikes one that is at rest, the body at rest returns the blow 
with equal force., 

This is illustrated by the well known fact, that if two per¬ 
sons strike their heads together, one being in motion, and 
the other at rest, they are both equally hurty 

156. The philosophy of action and re-action is finely il¬ 

lustrated by a number of ivory balls, suspended by threads, 
as in fig. 8, so as to touch Fl §- 8 - 

each other. If the ball a be 
drawn from the perpendicu¬ 
lar, and then let fall, so as to 
strike the one next to it, the 
motion of the falling ball will 
be communicated through 
the whole series, from one to 
the other. None of the balls, 
except /, will, however^ap- 
pear to move. This will be 
understood, when we consid¬ 
er that the re-action of -b, is 
just equal to the action of a, 
and that each of the other 
balls, in like manner, acts, 
and re-acts, on the other, un¬ 
til the motion of a arrives at f, which, having no impedi¬ 
ment, or nothing to act upon, is itself put in motion. It is, 
therefore, re-action, which causes all the balls, except /, to 
remain at rest., 

157. It is by a modification of the same principle, that 
rockets are impelled through the air. The stream of ex¬ 
panded air, or the fire, which is emitted from the lower end 
of the rocket, not only pushes against the rocket itself, but 
against the atmospheric air, which, re-acting against the air 
so expanded, sends the rocket along. 

158. It was on account of not understanding the princi¬ 
ples of action and re-action, that the man undertook to make 
a fair wind for his pleasure boat, to be used whenever he 
wished to sail. He fixed an immense bellows in the stem 



** How is this illustrated? 1 When one of the ivory balls strikes the other, why 
does the most distant one only move ? On what principle are rockets impelled 
through the air ? In the experiment with the boat and bellows, why did the 
boat move backwards ? Why would it not have moved at all, had the sail re 
ceived all the wind from the bellows ? 









REFLECTED MOTION. 


41 


of his boat, not doubting that the wind from it would carry 
him along. But on making the experiment, he found that 
his boat went backwards instead of forwards. The reason 
is plain. The re-action of the atmosphere on the stream of 
wind from the bellows, before it reached the sail, moved the 
boat in a contrary direction^ Had the sails received the 
whole force of the wind from the bellows, the boat would not 
have moved at all, for then, action and re-action would have 
been exactly equal, and it would have been like a man’s at¬ 
tempting to raise himself over a fence by the straps of his 
booi 



REFLECTED MOTION. 


159. It has been stated that all bodies, when once set in 
motion, would continue to move straight forward, until some 
impediment, acting in a contrary direction, should bring 
them to rest; continued motion without impediment being a 
consequence of the inertia of matter. 

160. Such bodies are supposed to be acted upon by a sin¬ 
gle force, and that in the direction of the line in which they 
move. Thus, a ball sent out of a gun, or struck by a bat, 
turns neither to the right nor left, but makes a curve to¬ 
wards the earth, in consequence of another force, which is 
the attraction of gravitation, and by which, together with 
the resistance of the atmosphere, it is finally brought to the 
ground. 

161. The kind of motion now to be considered, is that 
which is produced when bodies are turned out of a straight 
line by some force, independent of gravity. 

162rA single force, or impulse, sends the body directly 
forward but another force, not exactly coinciding with this, 
will give it a new direction, and bend it out of its former 
course. 

163. If, for instance, two moving bodies strike each other 
obliquely, they will both be thrown out of the line of their 
former direction. This is called [reflected motion^ becauseyt 
observes the same laws as reflected light) L j ^ 

164. [The bounding of a ball; the skipping of a stone 
over the smooth surface of a pond; and the oblique direction 


Suppose a body is acted on, and set in motion by a single force, in what di¬ 
rection will it move ??>Whatis the motion called, when a body is turned out of 
a straight line by another force ? What illustrations can you give of reflected 
motion? What laws are observed in reflected motion ?. Supposes ball to be 
thrown on the floor in a certain direction, what rule will it observe in relxMind- 
ing? 


4* 




42 


COMPOUND MOTION. 


of an apple, when it touches a limb in its fall, are examples 

of reflected motion. ! . . c . 

165. By experiments on this kind of motion, it is tound 
that moving bodies observe certain laws, in respect to the di¬ 
rection they take in rebounding from any impediment they 
happen to strike. Thus a ball, striking on the floor, or wall 
of a room, makes the same angle in leaving the point where 
it strikes, that it does in approaching 

166. Suppose a, Fig. 9. 

b , fig. 9, to be a 

marble slab, or floor, 
and c,tobe an ivory 
ball,which has been 
thrown towards the 
floor in the direction 
of the line c e ; it 

will rebound in the direction of the line e d , thus making 
the two angles f and g exactly equal) 

167. If the ball approached the 'floor under a larger or 
smaller 



its rebound would 


observe 
Fig. 10. 


the same rule. 


angle, 
Thus, if it fell in the 
line A A, fig. 10, its re¬ 
bound would be in 
the line A i, and if it 
was dropped perpen¬ 
dicularly from l to A, 
it would return in 
the same line to l. 
The angle which 
the ball makes with 
the perpendicular l 


A, in its approach to the floor, is called ihtfangle of incidence) 


and that which it makes in departing from the floor with the 
same line, is called the angle of refection, and these angles 

nvo oliiroxrc* onnol tA oooA AfViov 1 



are always equal to each other.* 


jj 37 

COMPOUND MOTION. 




168. Compound motion is that motion which is produced by 
two or more forces, acting in different directions , on the same 
body , at the same time v This will be readily understood by 
a diagram. J 


What is tne angle called, which the bull makes in approaching the floor f 
What is the angle called, which it makes in leaving the floor ? What is tne 
difference between these angles ? What is compound motion ? 


\ 






COMPOUND MOTION. 


43 


/ 


169. ^Suppose the ball Fig. 11. 

a, fig. 11, to be moving 
with a certain velocity 
in the line b c, and sup¬ 
pose that at the instant 
when it came to the point 
a, it should be struck 
with an equal force in 
the direction of d e , as it 
cannot obey the direc¬ 
tion of both these forces, 
it will take a course be¬ 
tween them, and fly off 
in the direction of f 

170. The reason of 
this is plain. The first force would carry the ball from b to 
c ; the second would carry it from d to e ; and these two for¬ 
ces being equal, gives it a direction just half way between 
the two. and therefore it is sent towards f) 

171. The line a f is called the diagonal of the square , and 
results from the cross forces, b and d, being equal to each 
other/- \If one of the moving forces is greater than the other, 
then the diagonal line will be lengthened in the direction of 
the greater force, and instead of being the diagonal of a 
square, it will become the diagonal of a parallelogram, or 
oblong square. 

172. Suppose the 
force in tlie direction of 
a b , should drive the 
ball with twice the ve¬ 
locity of the cross force 
c d, fig. 12, then the ball 
would go twice as far 
from the line c d , as from 
the line b a, and e f 
would be the diagonal of 
a parallelogram whose 
length is double its breadth. 

173. Suppose a boat, in crossing a river, is rowed forward 
at the rate of four miles an hour, and the current of the river 


Suppose a ball, moving with a certain force, to be struck crosswise with the 
same force, in what direction will it move? Suppose it to be struck with half 
its former force, in what direction will it move ? What is the line a /, fig. 11, 
called ? What is the line e f, fig. 12, called ? How are these figures illustrated ? 


Fig. 12. 


C 















44 


CIRCULAR MOTION. 


is at the same rate, then the two cross forces will he equal, 
and the line of the boat will be the diagonal of a square, as 
in fig. 11. But if the current be four miles an hour, and the 
progress of the boat forward only two miles an hour, then 
the boat will go down stream twice as fast as she goes across 
the river, and her path will be the diagonal of a parallelo¬ 
gram, as in fig. 12, and therefore to make the boat pass di¬ 
rectly across the stream, it must be rowed towards some point 
higher up the stream than the landing place; a fact well 
known to boatmen. | 

174. Circus Rider. -4Those who have seen feats of horse¬ 
manship at the circus, are often surprised that when the man 
leaps directly upward, the horse does not pass from under 
him, and that in descending he does not fall behind the ani¬ 
mal. But it should be considered that, on leaving the sad¬ 
dle, the body of the rider has the same velocity as that of the 
horse; nor does his leaving the horse by jumping upward, in 
any degree diminish his velocity in the same direction; his 
motion being continued by the impulse he had gained from 
the horse. In this case, the body of the man describes the 
diagonal of a parallelogram, one side of which is in the di¬ 
rection of the horse’s motion, and the other perpendicularly 
upward, in the direction in which he makes the leap. 

CIRCULAR MOTION. 

175. Circular motion is the motion of a body in a ring, or 
circle , ahd'is,produced by the action of two forces. By one of 
thesefforces , the moving body tends to fiy off in a straight line , 
while by the other it is drawn towards the centre, and thus it 
is made to revolve, or move round in a circle/ 

176. ''The force by which a body tends to go off in a 
straight line, is called the centrifugal force ; that which keeps 
it from flying away, and draws it towards the centre, is called 
the centripetal force) 

177. Bodies moving in circles are constantly acted upon 
by theselwo forces. If the centrifugal force should cease, 
the moving body would no longer perform a circle, but would 
directly approach the centre of its own motion.,' If the cen¬ 
tripetal force should cease, the body would instantly begin 
to move off in a straight line, this being, as we have explain- 


Why does the leaping circus rider form the diagonal of a parallelogram ? 
What is circular motion ? How is this motion produced ? What is the cen¬ 
trifugal force ? What is the centripetal force ? Suppose the centrifugal force 
should cease, in what direction would the body move T 



CIRCULAR MOTION. 


45 


ed, the direction which all bodies take when acted on by a 
single force. / 

178.1 This will be obvi¬ 
ous by ng. 13. Suppose a 
to be a cannon ball, tied with 
a string to the centre of a 
slab of smooth marble, and 
suppose an attempt be made 
to push this ball with the 
hand in the direction of b; 
it is obvious that the string 
would prevent its going to 
that point; but would keep 
it in the circle. In this case 
the string is the centripetal 
force. 

179. Now suppose the ball to be kept revolving with ra¬ 
pidity, Jits velocity and weight will occasion its centrifugal 
force j knd if the string were cut, when the ball was at the 
point'c, for instance, this force would carry it off in the line 
towards b. - 

I8%^he greater the velocity with which a body moves 
round" in a circle, the greater will be the force with which it 
will fly off in a right line. 

181. \jhus, when one wishes to sling a stone to the great¬ 
est distance, he makes it whirl round with the greatest pos¬ 
sible rapidity, before he lets it go. Before the invention of 
other warlike instruments, soldiers threw stones in this man¬ 
ner, with great force and dreadful effects. ) 

182. The line about which a body revolves, is called its 
accis of motion) (The point round which it turns, or on which 
it rests, is called the centre of motion. ),In fig. 13 the point d, 
to which the string is fixed, is the centre of motion. In the 
spinning-top, a line through the centre of the handle to the 
point on which it turns, is the axis of motion^ 

183. In the revolution of a wheel, that part which is at 
the greatest distance from the axis of motion, has the great¬ 
est velocity, and, consequently, the greatest centrifugal force. 


Suppose the centripetal force should cease, where would the body go ? Ex¬ 
plain fig. 13 . What constitutes the centrifugal force of the body moving round 
in a circle ? How it this illustrated ? W'hat is the axis of motion ? What is 
the centre of motion ? Give illustrations. What part of a revolving wheel 
has the greatest centrifugal force ? 






46 


CENTRE OF GRAVITY 


184. \ Suppose the wheel, fig. 

14, to revolve a certain num¬ 
ber of times in a minute, the 
velocity of the end of the arm 
at the point a, would be as 
much greater than its middle 
at the point b, as its distance 
is greater from the axis of mo¬ 
tion, because it moves in a 
larger circle, and consequent¬ 
ly the centrifugal force of the 
rim c, would, in like manner, 
be as its distance from the 
centre of motion. 

185. Large wheels, which are designed to turn with great 
velocity, must, therefore, be made with corresponding strength, 
otherwise the centrifugal force will overcome the cohesive 
attraction, or the strength of the fastenings, in which case 
the wheel will fly in pieces. This sometimes happens to the 
large grindstones used in gun factories, and the stone either 
flies away piece-meal, or breaks in the middle, to the great 
danger of the workmen.' 

186. \Were the diurnal velocity of the earth about seven¬ 
teen times greater than it is, those parts at the greatest dis¬ 
tance from its axis would begin to fly off in straight lines, 
as the water does from a grindstone when it is turned rap- 



Fig. 14. 



CENTRE OF GRAVITY. 


187. The centre of gravity, in any body or system of bodies, 
is that point upon which the body, or system of bodies, acted 
upon only by gravity, will balance itself in all positions'./ 

188. '{J’he centre of gravity, in a wheel made entirely of 
wood, and of equal thickness, would be exactly in the mid¬ 
dle, or in its ordinary centre of motion^ But if one side of 
the wheel were made of iron, and the ; other part of wood, 
its centre of gravity would be changed to some point, aside 
from the centre of the wheel. 

* 


Why must large wheels, turning with great velocity, be strongly made ? 
What would be the consequence, were the velocity of the earth 17 times great¬ 
er than it is ? Where is the centre of gravity in a body ? Where is the cen¬ 
tre of gravity in a wheel, made of wood ? If one side is made of wood, and 
the other of iron, where is the centre ? 












47 


CENTRE OF GRAVITY. 


Fig. 15. 


189. tThus, the, centre of gravity 
in the wooden wheel, fig. 15, would 
be at the axis on which it turns ; 
but were the arm a , of iron, its cen¬ 
tre of motion and of gravity would 
no longer be the same, but while 
the centre of motion remained as 
before, the centre of gravity would 
fall to the point a. Thus the cen¬ 
tre of motion and of gravity, 
though often at Jhe same point, are 
not always so. j 

190. ^hen the body is shaped irregularly, or there are 
two or more bodies connected, the centre of gravity is the 
point on which they will balance,without falling,) 

19 L/ If the two balls, a and 1, - Fig! 16. 

fig. lfe, weigh each four pounds, ~ & h 

the centre of gravity will be a point 
on the bar equally distant from 
each. 



192. But if one of the balls be 
heavier than the other, then the cen¬ 
tre of gravity will, in proportion, ap¬ 
proach the larger ball. Thus, in 
fig. 17, if c weighs two pounds, and 
d eight pounds, the centre of gravi¬ 
ty will be four times the distance from c that it is from d. 

193. In a body of equal thickness, as a board, or a slab 
of marble; but otherwise of an irregular shape, the centre of 
gravity may be found by suspending it, first from one point, 
and then from another, and marking, by means of a plumb 
line, the perpendicular ranges from the point of suspension. 
The centre of gravity will be the point where these two lines 
cross each other./ 

Thus, if the irregular shaped piece of board, fig. 18, be 
suspended by making a hole through it at the point a, and at 
the same point suspending the plumb line c , both board and 
line will hang in the position represented in the figure. 
Having marked this line across the board, let it be suspend¬ 
ed again in the position of fig. 19, and the perpendicular line 
again marked. The point where these lines cross each oth¬ 
er, is the centre of gravity, as seen by fig. 20./' 


Fig. 17. 



Is the centre of motion and of gravity always the same ? When two bodies 
are connected, as by a bar between them, where is the centre of gravity J 







48 


CENTRE OF GRAVITY. 


Fig. 18. 



Fig. 19. 



Fig. SO. 



194. It is often of great consequence, in the concerns oi 
life, that the subject of gravity should be well considered, 
since the strength of buildings, and of machinery, often de¬ 
pends chiefly on the gravitating point. 

195. Common experience teaches, that a tall object, with 
a narrow base, or foundation, is easily overturned ; but com¬ 
mon experience does not teach the reason, for it is only by 
understanding principles, that practice improves experiment. 

196. 'sAn upright object will fall to the ground, when it 
leans so 'much that a perpendicular line from its centre of 
gravity falls beyond its base. A tall chimney, therefore, 
with a narrow foundation, such as are commonly bujlt at the 
present day, will fall with a very slight inclination./ 

197\£Tow, in falling, the centre of gravity passes through 
the part of a circle, the centre of which is at the extremity 
of the base on which the body stands. \ This will be com¬ 
prehended by fig. 21. 

198.\ Suppose the figure to be a 
block of marble, which is to be turn¬ 
ed over, by lifting at the corner a , the 
corner d would be the centre of its 
motion, or the point on which it would 
turn. The centre ofgravity, c, would, 
therefore, describe the part of a circle, 
of which the corner, d , is the centre. 

199\ It will be obvious, after a little consideration, that 
the greatest difficulty we should find in turning over a square 
block of marble, would be, in first raising up the centre of 
gravity, for the resistance will constantly become less, in 
proportion as the point approaches a perpendicular line over 

In a board of irregular shape, by what method is the centre of gravity found ? 
In what direction must the centre of gravity be from the outside of the base 
before the object will fall ? In falling, the centre of gravity passes through 
part of a circle ; where is the centre of this circle ? In turning over a body 
why does the force required constantly beoome less and less ? 










CENTRE OF GRAVITY. 


49 


the corner d , which, having passed, it will fall by its own 

gravity.,! 

200. The difficulty of turning over a body of a particular 
form, will be more strikingly illustrated by the figure of a 
triangle, or low pyramid. 

201. /In fig. 22, the centre of gravity Fig. 22 - 

is so low, and the base so broad, that 
in turning it over, a great proportion 
of its whole weight must be raised. 

Hence we see the firmness of the pyra¬ 
mid in theory, and experience proves 
its truth ; for buildings are found to 
withstand the effects of time, and the 
commotions of earthquakes, in proportion as they approach 
this figure. ) 

The most ancient monuments of the art of building, now 
standing, the pyramids of Egypt, are of this form. 

202. (When a ball is rolled on a horizontal plane, the cen¬ 
tre of gravity is not raised, but moves in a straight line paral¬ 
lel to the surface of the plane on which it rolls, and is con¬ 
sequently always directly over its centre of motion.) 

203.lSuppose, fig. 23, a is the Fig 23 f 

plane oIT which the ball moves, 
b the line on which the centre 
of gravity moves, and c a plumb 
linei showing that the centre of 
gravity must always be exactly 
over the centre of motion, when 
the ball moves on a horizontal 
plane—then we shall see the 
reason why a ball moving on 
such a plane, will rest with equal firmness in any position, 
and why so little force is required to set it in motion. For 
in no other figure does the centre of gravity describe a hori¬ 
zontal line over that of motion, in whatever direction the 
body is moved. \ 

204\_If the plane is inclined downwards, the ball is in¬ 
stantly thrown into motion, because the centre of gravity 
then falls forward of that of motion, or the point on which 
the ball rests} 


Why is there less force required to overturn a cube, or square, than a pyra¬ 
mid of the same weight ? When a ball is rolled on a horizontal plane, in what 
direction does the centre of gravity move ? Explain fig. 23. Why does a ball 
on a horizontal plane rest equally well in all positions ? Why does it move 
with little force ? If the plena is inclined downwards, why does the ball ml' 
in that direction ? 




o 








50 


CENTRE OF GRAVITY. 


20f*. This is explained by fig. 
24, where a is the point on which 
the ball rests, or the centre of 
motion, c the perpendicular line 
from the centre of gravity, as 
shown by the plumb weight c. 

If the plane is inclined upward, 
force is required to move the ball 
in that direction,^because the cen¬ 
tre of gravity thenfalls behind that 
of motion, and therefore the centre 
of gravity has to be constantly 
liftedA This is also shown by fig. 
24, onfy considering the ball to be 
plane, instead of down it. 


Fig. 24. 



moving up the inclined 


200. From these principles, it will be readily understood 
why so much force is required to roll a heavy body, as a 
hogshead of sugar, for instance, up an inclined plane. The 
centre of gravity falling behind tnat of motion, the weight is 
constantly acting against the force employed to raise the 


207. \One of the best illustrations of this 
subject may be made by a number of 
square blocks of wood, placed on each 
other as in fig. 25 ; forming a leaning 
tower. Where five blocks are placed in 
this position, the point of gravity is near 
the centre of the third block, and is within 
the base as shown by the plumb line. But 
on adding another block the gravitating 
point falls beyond the base, and the whole 
will now fall by its own weight. The 
student having such blocks, (and they may 
be picked up about any joiner’s shop,) will 
convince himself, that however carefully his 
leaning tower is laid up, it will not stand 
when the centre of gravity falls an inch or 
two beyond the support. 

From what has been stated, it will be 


Fig 25. 



Why is force required to move a ball up an inclined plane ? What is the 
danger that a body will fall proportioned to ? Why is a body, shaped like fig. 
25, more easily thrown down, than one shaped like fig. 26 ? Hence, in riding 
in a carriage, how is the danger of upsetting proportioned ? How may the 
point of gravity be found by means of a number of square blocks ? 






CENTRE OF GRAVITY. 


51 


Fig. 20. 


the danger that a body will fall, is in pro¬ 
portion to the narrowness of its base, com¬ 
pared with the height of the centre of 
gravity above the base, j 

208. /Thus, a tall boay, shaped like fig. 

26, wilMall, if it leans but very slightly, 
for the centre of gravity being far above 
the base,' at a , is brought over the centre 
of motion, b , with little inclination, as shown 
by the plumb line. Whereas, a body 
shaped like fig. 27, will not fall until it 
leans much more, as again shown by the 
direction of the plumb line/ 

209. (We may learn,^from these com- 
parisons/lhat it is more dangerous to ride 
in a high carriage than a low one, in pro¬ 
portion to the elevation of the vehicle, and 
the nearness of the wheels to each other, or 
in proportion to the narrowness of the base, 
and the height of the centre of gravity. 

A load of hay, fig. 28, upsets where 
the road raises one wheel but little 
higher than the other, because it 
is high, and broader on the top 
than the distance of the wheels 
from each other; while a load of 
stone is very rarely turned over, 
because the centre of gravity is 
near the earth, and its weight 
between the wheels, instead of 
being far above them/ 

210. In man the centre of grav¬ 
ity is between the hips, and hence, 
were his feet tied together^ and 
his arms tied to his sides ,M very 

slight inclination of his body would carry the perpendicular 
of his centre of gravity beyond the base, and he would fall. 
But when his limbs are free to move, he widens his base, 
and changes the centre of gravity at pleasure, by throwing 
out his arms, as circumstances require. | 

211. When a man runs, he inclines forward, so that the 
centre of gravity may hang before his base, and in this 

Where is the centre of man’s gravity ? Why will a man fall with a slight 
inclination, when his feet and arms are tied ? 










CENTRE OF GRAVITY. 


52 

^ » 

position he is obliged to keep his feet constantly advancing, 
otherwise he would fall forward. 

212. A man standin r on one foot, cannot throw his body 
forward without at th * same time throwing his other foot 
backward, in order to 1? 3 ep the centre of gravity within the 
base. 

213. /A man, therefore, standing with his heels against a 
perpendicular wall, cannot stoop forward without falling, be¬ 
cause the wall prevents his throwing any part of his body 
backward. A person little versed in such things, agreed to 
pay a certain sum of money for an opportunity of possessing 
himself of double the sum, by taking it from the floor with 
his heels against the wall. The man, of course, lost his 
money, for in such a posture, one can hardly reach lower 
than his own knee. 

214. '\The base on which a man is supported, in walking 
or standing, is his feet, and the space between them. By 
turning the toes out, this base is made broader, without 
taking much from its length, and hence persons who turn 
their toes outward, not only walk more firmly, but more 
gracefully, than those who turn them inward. 

215. 'vjn consequence of the upright positionAif man, he is 
constantly obliged to employ some exertion to keep his bal¬ 
ance. This seems to be the reason why children learn to 
walk with so much difficulty, for after they have strength 
to stand, it requires considerable experience so to balance 
the body as to set one foot before the other without falling^ 

216. ; By experience in the art of balancing, or of keeping 
the centre of gravity in a line over the base, men sometimes - 
perform things, that, at first sight, appear altogether beyond 
human power, such as dining with the table ajid chair 
standing on a single rope, dancing on a wire, &c. 

217. \ No form, under which matter exists, escapes the 
generalTaw of gravity, and hence vegetables, as well as an¬ 
imals, are formed with reference to the position of this centre, 
in respect to the base. 

It is interesting, in reference to this circumstance, to ob¬ 
serve how exactly the tall trees of the forest conform to this 
law. 


Why cannot one who stands with his heels against a wall stoop forward ? 
Why does a person walk most firmly, who turns his toes outward ? Why 
does not a child walk as soon as he can stand ? In what does the art of bal¬ 
ancing, or walking on a rope, consist ? What is observed in the growth 01 
the trees of the forest, in respect to the laws of gravity ? What effect does 
inertia have on bodies at rest ? What effect does it have on bodies in motion ? 



CENTRE OF INERTIA. 


53 

218. The pine, which grows a hundred feet high, shoots 
up with as much exactness, with respect to keeping its cen¬ 
tre of gravity within the base, as though it had been directed 
by the plumb line of a master builder. Its limbs towards 
the top are sent off in conformity to the same law ; each One 
growing in respect to the other, so as to preserve a due bal¬ 
ance between the whole. 

219. It may be observed, also, that where many trees 
grow near each other, as in thick forests, and consequently 
where the wind can have but little effect on each, that they 
always grow taller than when standing alone on the plain. 
The roots of such trees are also smaller, and do not strike 
so deep as those trees standing alone. A tall pine, in the 
midst of the forest, would be thrown to the ground by the 
first blast of wind, were all those around it cut away. 

Thus the trees of the forest not only grow so as to pre¬ 
serve their centres of gravity, but actually conform, in a 
certain sense, to their situation, j 

CENTRE OF INERTIA. 

220. It will be remembered that inertia (22) is one of the 
inherent, or essential properties of matter, and that it is in 
consequence of this property, when bodies are at rest, that 
they never move without the application of force, and when 
once in motion, that they never cease moving without some 
external cause. 

221. Now, inertia, though like gravity, it resides equally 
in every particle of matter, must have, like gravity, a centre 
in each particular body, and this centre is the same with 
that of gravity. 

222. In a bar of iron, six feet long and two inches square, 
the centre of gravity is just three feet from each end, or ex¬ 
actly in the middle. If, therefore, the bar is supported at 
this point, it will balance equally, and because there are 
equal weights on both ends, it will not fall. 

Now suppose a bar should be raised by raising up the 
centre of gravity, then the inertia of all its parts would be 
overcome equally with that of the middle. The centre of 
gravity is, therefore, the centre of inertia. 

223. The centre of inertia, being that point which, being 


Is the centre of inertia, and that of gravity, the same ? Where is the centre 
of inertia in a body, or a system of bodies ? Why is the point of inertia 
changed, by fixing different weights to the ends of the iron bar ? 

5* 



54 


EQUILIBRIUM. 


lifted, the whole body is raised, is not, therefore, always at 
the centre of the body. 

224. Thus, suppose the same 
oar of iron, whose inertia was 
overcome by raising the centre, to 
have balls of different weights 
attached to its ends; then the 
centre of inertia would no longer 
remain in the middle of the bar, but would be changed to 
the point a, fig. 29, so that to lift the whole, this point must 
be raised, instead of the middle, as before. 

EQUILIBRIUM. 

225. When two forces counteract , or balance each other , 
they are said to be in equilibrium. 

226. It is not necessary for this purpose, that the weights 
opposed to each other should be equally heavy, for we have 
just seen that a small weight, placed at a distance from the 
centre of inertia, will balance a large one placed near it. 
To produce equilibrium, it is only necessary that the weights 
on each side of the support should mutually counteract each 
other, or if set in motion, that their momenta should be equal. 

A pair of scales are in equilibrium when the beam is in 
a horizontal position. 

227. To produce equilibrium in solid bodies, therefore , it is 
only necessary to support the centre of inertia , or gravity . 

228. If a body, or several bo- Fig. 30 . 

dies, connected, be suspended 
by a string, as in fig. 30, the 
point of support is always in a 
perpendicular line above the cen¬ 
tre of inertia. The plumb line 
d , cuts the bar connecting the 
two balls at this point. Were 
the two weights in this figure 
equal, it is evident that the hook 
or point of support, must be in the middle of the string, to 
preserve the horizontal position. 

When a man stands on his right foot, he keeps himself in 
equilibrium, by leaning to the right, so as to bring his cen- 


What is meant by equilibrium ? To produce equilibrium, must the weights 
be equal ? When is a pair of scales in equilibrium ? When a body is sus¬ 
pended by a string, where must the support be with respect to the point of 
inertia? 









CURVILINEAR MOTION. 


55 


dre of gravity in a perpendicular line over the foot on which 
he stands. 

CURVILINEAR, OR BENT MOTION. 

229. We have seen that a single force acting on a body, 
(162,) drives it straight forward, and that two forces acting 
crosswise, drive it midway between the two, or give it a di¬ 
agonal direction, (169.) 

230. Curvilinear motion differs from both these, the direc¬ 
tion of the body being neither straight forward, nor diagonal, 
but through a line which is curved. 

This kind of motion may be in any direction, but when it 
is produced in part by gravity, its direction is always towards 
the earth. 

231. A stream of water from an aperture in the side of a 
vessel, as it falls towards the ground, is an example of a 
curved line ; and a body passing through such a line, is said 
to have curvilinear motion. Any body projected forward, as 
a cannon ball or rocket, falls to the earth in a curved line. 

232. It is the action of gravity across the course of the 
stream, or the path of the ball, that bends it downwards, and 
makes it form a curve. The motion is therefore the result of 
two forces, that of projection, and that of gravity. 

233. The shape of the curve will depend on the velocity 
of the stream or ball. When the pressure of the water is 
great, the stream, near the vessel, is nearly horizontal, because 
its velocity is in proportion to the pressure. When a ball 
first leaves the cannon, it describes but a slight curve, be¬ 
cause its projectile velocity is then greatest. 

The curves described by jets of water, under different de¬ 
grees of pressure, are readily illustrated by tapping a tall ves¬ 
sel in several places, one above the other. 

234. Suppose fig. 31 to be such a vessel, filled with water, 
and pierced as represented. The streams will form curves 
differing from each other, as seen in the figure. Where 
the projectile force is greatest, as from the lower orifice, 
the stream reaches the ground at the greatest distance from the 
vessel, this distance decreasing, as the pressure becomes less 
towards the top of the vessel. The action of gravity being 
always the same, the shape of the curve described, as just 

What is meant by curvilinear motion ? What are examples of this kind of 
motion ? What two forces produce this motion ? On what does the shape of 
the curve depend ? How are the curves described by jets of water illustrated l 
What difference is there in respect to the time taken by a body to reach the 
ground, whether the curve be great or small ? Why do bodies forming differ 
ent curves from the same height, reach the ground at the same time 7 



56 


CURVILINEAR MOTION. 


stated, must depend on the 
velocity of the moving 
body ; but whether the pro¬ 
jectile force be great or 
small, the moving body, if 
thrown horizontally, will 
reach the ground from the 
same height in the same 
time. 

235. This, at first thought, 
would seem improbable, for, 
without consideration, most 
persons would assert, very 
positively, that if two can¬ 
non were fired from the same 
spot, at the same instant, and in the same direction, one of 
the balls falling half a mile, and the other a mile distant, 
that the ball which went to the greatest distance, would 
take the most time in performing its journey. 

236. But it must be remembered, that the projectile force 
does not in the least interfere with the force of gravity. A 
ball flying horizontally at the rate of a thousand feet per 
second, is attracted downwards with precisely the same force 
as one flying only a hundred feet per second, and must 
therefore descend the same distance in the same time. 

237. The distance to which a ball will go, depends on the 
force of impulse given it the first instant, and consequently 
on its projectile velocity. If it moves slowly, the distance 
will be short—if more rapidly, the space passed over will 
be greater. It makes no difference, then, in respect to the 
descent of the ball, whether its projectile motion be fast, or 
slow, or whether it moves forward at all. 

238. This is demonstrated by experiment. Suppose a 
cannon to be loaded with a ball, and placed on the top of a 
tower, at such a height from the ground, that it would take 
just three seconds for a cannon ball to descend from it to the 
ground, if let fall perpendicularly. Now suppose the can¬ 
non to be fired in an exact horizontal direction, and at the 
same instant the ball to be dropped towards the ground. 

Suppose two balls, one flying at the rate of a thousand, and the other at the rate 
of a hundred feet per second, which would descend most during the second ? 
Does it make any difference in respect to the descent of the ball, whether it 
has a projectile motion or not ? Suppose, then, one ball be fired from a can¬ 
non, and another let fall from the same height at the same instant, would they 
Doth reach the ground at the same time 1 


Fig. 31. 






CURVILINEAR MOTION. 


57 

J hey will both reach the ground at the same instant, provi¬ 
ded its surface be a horizontal plane from the foot of the tow¬ 
er to the place where the projected ball strikes. 

239. This will be made plain by fig. 32, where a is the 
perpendicular line of the descending ball, c b the curvilinear 
path of that projected from the cannon, and d, the horizon¬ 
tal line from the foot of the tower. 

Fig. 32. 



The reason why the two balls reach the ground at the 
same time, is easily comprehended. 

240. During the first second, suppose that the ball which 
is dropped, reaches 1; during the next second it falls to 2; 
and at the end of the third second it strikes the ground. 
Meantime, the ball shot from the cannon is projected for¬ 
ward with such velocity as to reach 4 in the same time that 
the other is falling to 1. But the projected ball falls down¬ 
ward exactly as fast as the other, for it meets the line 1, 4, 
which is parallel to the horizon, at the same instant. Du¬ 
ring the next second, the projected ball reaches 5, while the 
other arrives at 2; and here again they have both descend¬ 
ed through the same downward space, as is seen by the line 
2, 5, which is parallel with the other. During the third sec¬ 
ond, the ball from the cannon will have nearly spent its pro¬ 
jectile force, and, therefore, its motion downward will be 
greater, while its motion forward will be less than before. 
The reason of this will be obvious, when it is considered, that 
in respect to gravity, both balls follow exactly the same law, 
and fall through equal spaces in equal times. Therefore, as 
the falling ball descends through the greatest space during 
the last second, so that from the cannon, having now a less 


Explain fig. 32, showing the reason why the two balls will reach the ground 
at the same time ? Why does the ball approach the earth more rapidly in the 
*ast part of the curve, than in the first part ? 













CURVILINEAR MOTION. 


58 


projectile motion, its downward motion is more direct, and 
like all falling bodies, its velocity is increased as it approach¬ 
es the earth. 

241. From these principles it may be inferred, that tho 
horizontal motion of a body through the air, does not in the 
least interfere with its gravitating motion towards the earth, 
and, therefore, that a rifle ball, or any other body projected 
forward horizontally, will reach the ground in exactly the 
same period of time, as one that is let fall perpendicularly 
from the same height. 

242. The two forces acting on bodies which fall through 
curved lines, are the same as the centrifugal and centripetal 
forces, already explained; the centrifugal, in case of the 
ball, being caused by the powder—the centripetal, being the 
action of gravity. 

Now, it is obvious, that the space through which a can¬ 
non ball, or any other body, can be thrown, depends on the 
velocity with which it is projected; for the attraction of 
gravitation, and the resistance of the air, acting perpetually, 
the time which a projectile can be kept in motion, through 
the air, is only a few moments. 

243. If, however, the projectile be thrown from an elevated 
situation, it is plain that it would strike at a greater distance 
than if thrown on a level, because it would remain longer in 
the air. Every one knows that he can throw a stone to a 
greater distance, when standing on a steep hill, than when 
standing on the plain below. 


244. Suppose the 
circle, fig. 33, to be 
the earth, and a, a 
high mountain on its 
surface. Suppose that 
this mountain reaches 
above the atmosphere, 
or is fifty miles high, 
then a cannon ball 
might perhaps reach 
from a to 6, a distance 
of eighty or a hun¬ 
dred miles, because 
the resistance of the 
atmosphere being out 
of the calculation, it 
would have nothing 


Fig. 33. 

a 



CURVILINEAR MOTION. 


59 

10 contend with, except the attraction of gravitation. If, 
then, one degree of force, or velocity, would send it to b , an¬ 
other would send it to c: and if the force was increased three 
times, it would fall to d, and if four times, it would pass to 
e. If now we suppose the force to be about ten times greater 
than that with which a cannon ball is projected, it would 
riot fall to the earth at any of these points, but would con* 
tinue its motion, until it again came to the point a, the place 
from which it was first projected. It would now be in equi¬ 
librium, the centrifugal force being just equal to that of 
gravity, and therefore it would perform another and another 
revolution, and so continue to revolve around the earth per¬ 
petually. 

245. Bonaparte, it is said, by elevating the range of his 
shot, bombarded Cadiz from the distance of five miles. Per¬ 
haps, then, from a high mountain, a cannon ball might be 
.hrown to the distance of six or seven miles. 

246. The reason why the force of gravity will not ulti¬ 
mately bring it to the earth, is, that during the first revolu¬ 
tion, the effect of this force is just equal to that exerted in 
any other revolution, but neither more nor less ; and, there¬ 
fore, if the centrifugal force was sufficient to overcome this 
attraction during one revolution, it would also overcome it 
during the next. It is supposed, also, that nothing tends to 
affect the projectile force except gravity, and the force of 
this attraction would be no greater during any other revolu¬ 
tion than during the first. 

247. In other words, the centrifugal and centripetal forces 
are supposed to be exactly equal, and to mutually balance 
each other; in which case, the ball would be, as it were, 
suspended between them. As long, therefore, as these two 
forces continued to act with the same power, the ball would 
no more deviate from its path, than a pair of scales would 
lose their balance without more weight on one side than on 
the other. 

It ifj these two forces which retain the heavenly bodies in 
their orbits, and in the case we have supposed, our cannon 


What is the force called which throws a ball forward? What is that caUed, 
which brings it to the ground? On what does the distance to which a project¬ 
ed body may be thrown depend ? Why does the distance depend on the ve¬ 
locity? Explain fig. 33. Suppose the velocity of a cannon ball shot from a 
mountain 50 miles high, to be ten times its usual rate, where would it stop? 
When would this ball be in equilibrium ? Why would not the force of gravity 
ultimately bring the ball to the earth ? After the first revolution, if the two 
forces continued the same, would not the motion of the ball be perpetual. 



GUNNERY. 


60 

ball would become a little satellite, moving perpetually round 
the earth. 

GUNNERY 

248. The laws of projectiles above explained, apply to the 
science of gunnery, a subject which ever since the discovery 
of gunpowder has occasionally attracted the attention oi 
philosophers of the first rank. 

Any body, of whatever kind, when thrown into the atmos¬ 
phere becomes a projectile; and the art of gunnery consists 
in projecting solids with force and accuracy, towards objects 
at a distance. 

249. The first accurate series of experiments made on this 
subject were those of Mr. Benjamin Robins, published in 
1742. In this work, which is still considered one of much 
elegance and accuracy, the author treats fully of the resist¬ 
ance of the atmosphere, the force of gunpowder of different 
compositions, the advantages and defects of guns of various 
kinds, and indeed of nearly every thing relating to military 
projectiles. 

250. Another set of experiments on gunnery was made by 
Dr. Hutton in 1775, and repeated, or extended during several 
succeeding years. These authors, together with Dr. Greg¬ 
ory in 1815, appear to have exhausted the subject of gun¬ 
nery, as no experiments of much consequence have since 
been published. 

The works of Robins and Hutton at the present day ap¬ 
pear to afford the best data for the theory and practice of the 
science in question. 

251. Velocity of the Ball. —There are several methods 
of determining the velocity of the ball. Mr. Robins in¬ 
vented what is called the ballistic pendulum. This is a 
heavy, thick block of wood, so suspended as to swing freely 
about an axis, and into which the ball is fired. This be¬ 
ing too thick for it to pass through, the whole momen¬ 
tum of the ball is transferred to the block, and the degree of 
motion given it, shows what the momentum has been. 
Hence the relative weights of the ball and the wood being 
known, the velocity of the former is readily computed. 

252. Another method is, by means of the recoil of the gun. 
The principle involved here, is, that the explosive force of the 


What is projectile? In what does tn.- art of gunnery consist? At what 
time, and by whom were the first accurate experiments in gunnery made? 
By what methods are the velocities of balls determined ? 



GUNNERY. 


61 


powder communicates equal quantities of motion both to the 
ball aM the gun in opposite directions. Hence by suspend¬ 
ing the gun, loaded with weights, like a pendulum, the ex¬ 
tent of its arc of vibration will indicate the force of the 
charge of powder employed, and by knowing the weights of 
the gun and ball, the velocity of the latter is determined. 

253. From such data Dr. Hutton constructed the table 
below, the gun throwing an iron ball of one pound weight. 
It shows the quantity of powder, velocity of the ball, the 
range, or distance the ball was thrown, and the time. 


POWDER. 

VELOCITY PER SECOND. 

DISTANCE. 

TIME OF FLIGHT. 

Ounces. 

Feet. 

Feet. 

Seconds. 

2 

800 

4100 

9 

4 

1230 

5100 

12 

8 

1640 

6000 

14* 

12 

1680 

6700 

15* 


254. By other experiments, it is found that the velocity of 
the ball increases with the charge to a certain point, which 
is peculiar to each gun, and that from this point it diminishes 
as the charge is increased until the bore is quite full: hence 
overloading a gun, so far from increasing, diminishes the de¬ 
structive effects. 

255. It i3 found also, that there is no difference in the ve¬ 
locity of the ball caused by varying the weight of the gun. 

From the above table it may be seen that doubling the 
charge of powder from 2 to 4 ounces, increased the velocity 
from 800 to 1230 feet, while adding one-third from 8 to 12 
ounces, only gave an increase of velocity of 40 feet to the 
second. 

256. The greatest velocity of a ball ever observed was 
about 2000 feet at the moment when it left the gun, and to 
obtain this it was found that one-third more powder was re¬ 
quired than that which gave a velocity of 1600, or 1700 feet 
per second. 

257. Power and destruction .—The power of penetration 
which a ball has, is proportional to the square of its velocity 
hence, when the object is merely to penetrate, as in the 
breaching of a fortification, the greatest velocity should be 
given. But in naval engagements great velocities are not 


What is said of an increase or diminution of the force of the ball, by varying 
the charge of powder ? Does the weight of the gun vary the force of the ball '• 
What is the greatest velocity at which a ball can be thrown ? What is said 
the destructive effects of the ball ? 

6 








RESULTANT MOTION. 


02 

lli*i most destructive, the ball having just sufficient force to 
go through the ship’s side doing the most mischief. 

RESULTANT MOTION. 

258. Suppose two men to be sailing in two boats, each at 
the rate of four miles an hour, at a short distance opposite 
to each other, and suppose as they are sailing along in this 
manner, one of the men throws the other an apple. In re¬ 
spect to the boats, the apple would pass directly across from 
one to the other, that is, its line of direction would be perpen¬ 
dicular to the sides of the boats. But its actual line through 
the air would be oblique, or diagonal, in respect to the sides 
of the boats, because in passing from boat to boat, it is im¬ 
pelled by two forces, viz., the force of the motion of the boat 
forward, and the force by which it is thrown by the hand 
across this motion. 

259. This diagonal motion of the apple is called the re¬ 
sultant, or the resulting motion, because it is the effect, or 
result of two motions, resolved into one. Perhaps this will 
be more clear by fig. 34, where 
a b , and c d, are supposed to 
be the sides of the two boats, 
and the line e f that of the 
apple. Now the apple when 
thrown, has a motion with the 
boat at the rate of four miles 
an hour, from c towards d, and 
this motion is supposed to continue just as though it had 
remained in the boat. Had it remained in the boat during 
the time it was passing from e to/, it would have passed 
from e to h. But we suppose it to have been thrown at the 
rate of eight miles an hour in the direction towards g, and if 
the boats are moving south, and the apple thrown towards 
the east, it would pass in the same time, twice as far towards 
the east as it did towards the south. Therefore, in respect 
to the boats, the apple would pass in a perpendicular line 
from the side of one to that of the other, because they are 


Suppose two boats, sailing at the same rate, and in the same direction; if an 
apple be tossed from one to the other, what will be its direction in respect to 
the boats ? What would be its line through the air, in respect to the boats'? 
What is this kind of motion called? Why is it called resultant motion ? 
Explain fig. 34. Why would the line of the apple be actually perpendicular 
m respect to the boats, but oblique in respect to parallel lines drawn from where 
it was thrown, and where it struck ? 


Fig. 34. 









RESULTANT MOTION. 


63 


both in motion; but m respect to one perpendicular line, 
drawn from the point where the apple was thrown, and a 
parallel line with this, drawn from the point where it strikes 
the other boat, the line of the apple would be oblique. This 
will be clear, when we consider, that when the apple is 
thrown, the boats are at the points e and g , and that when it 
strikes, they are at h and /, these two points being opposite 
to each other. 

The line e f through which the apple is thrown, is called 
the diagonal of a parallelogram, as already explained under 
compound motion. 

260. On the above principle, if two ships, during a battle, 
are sailing before the wind at equal rates, the aim of the 
gunners will be exactly the same as though they stood still; 
whereas, if the gunner fires from a ship standing still, at 
another under sail, he takes his aim forward of the mark he 
intends to hit, because the ship would pass a little forward 
while the ball iq going to her. And so, on the contrary, if a 
ship in motion fires at another standing still, the aim must be 
behind the mark, because as the motion of the ball partakes 
of that of the ship, it will strike forward of the point aimed at. 

261. For the same reason, if a ball be dropped from the 
topmast of a ship under sail, it partakes of the motion of the 
ship forward, and will fall in a line with the mast, and strike 
the same point on the deck, as though the ship stood still. 

262. If a man upon the full run drops a bullet before him 
from the height of his head, he cannot run so fast as to over¬ 
take it before it reaches the ground. 

263. It is on this principle, that if a cannon ball be shot 
up vertically from the earth, it will fall back to the same 
point; for although the earth moves forward while the ball 
is in the air, yet as it carries this motion with it, so the ball 
moves forward also, in an equal degree, and therefore comes 
down at the same place. 

264. Ignorance of these laws induced the story-making 
sailor to tell his comrades, that he once sailed in a ship 
which went so fast, # that when a man fell from the mast¬ 
head, the ship sailed away and left the poor fellow to strike 
into the water behind her. 


How is this further illustrated ? When the ships are in equal motion, where 
does the gunner take his aim? Why does he aim forward of the mark when 
the other ship is in motion ? If a ship in motion fires at one standing still, 
where must be the aim ? Why, in this case, must the aim be behind the mark ? 
What other illustrations are given of resultant motion ? 



04 


PENDULUM. 


PENDULUM. 


*265. A pendulum is a heavy body, such as a piece of brass 
or lead, suspended by a wire or cord, so as to swing backwards 
and forwards. 

When a pendulum swings, it is said to vibrate ; and that 
part of a circle through which it vibrates, is called its arc. 

266. The times of the vibration of a pendulum are very 
nearly equal, whether it pass through a greater or less part 
of its arc. 



Suppose a and b , fig. 35, to be two pendulums of equal 
length, and suppose the weights of each are carried, the one 
to c, and the other to d , and both let fall at the same in¬ 
stant ; their vi¬ 
brations would Fig. 35. 

be equal in re¬ 
spect to time, 
the one pass¬ 
ing through its 
arc fromc to e, 
and so back 
again in the 
same time that 
the other pass¬ 
es from d to f 
and back again. 

The reason of this appears to be, that when the pendulum 
is raised high, the action of gravity draws it more directly 
downwards, and it therefore acquires, in falling, a greater 
comparative velocity than is proportioned to the trifling dif¬ 
ference of height. 

267. In the common clock, the pendulum is connected 
with wheel work, to regulate the motion of the hands, and 
with weights, by which the whole is moved. The vibra¬ 
tions of the pendulum are numbered by a wheel having thirty 
teeth, which revolves once in a minute. Each tooth, there¬ 
fore, answers to one vibration of the pendulum, and the wheel 
moves forward one tooth in a second. Thus the second hand 
revolves once in every sixty beats of the pendulum, and as 
these beats are seconds, it goes round once in a minute. I 'y 


What is a pendulum ? What is meant by the vibration of a pendulun ’ 
What is that part of a circle called, through which it swings ? Why does 
pendulum vibrate in equal time, whether it goes through a small or large pan 
of its arc ? Describe the common clock. How many vibrations has the pen¬ 
dulum in a minute ? 





PENDULUM. 


65 


the pendulum, the whole machine is regulated, for the clock 
goes faster, or slower, according to its number of vibrations 
in a given time. The number of vibrations which a pendu • 
lum makes in a given time, depends upon its length, because 
a long pendulum does not perform its journey to and from 
the corresponding points of its arc so soon as a short one. 

268. As the motion of the clock is regulated entirely by 
the pendulum, and as the number of vibrations are as its 
length, the least variation in this respect will alter its rate of 
going. To beat seconds, its length must be about thirty-nine 
inches. In the common clock, the length is regulated by a 
screw, which raises and lowers the weight. But as the rod 
to which the weight is attached, is subject to variations of 
length in consequence of the change of the seasons, being 
contracted by cold and lengthened by heat, the common 
clock goes faster in winter than in summer. 

269. Various means have been contrived to counteract 
the effects of these changes, so that the pendulums may con¬ 
tinue the same length the whole year. Among inventions 
for this purpose, the gridiron pendulum is considered the 
best. It is so called, because it consists of several rods of 
metal connected together at each end. 

270. The principle on which this pendulum is constructed, 
is derived from the fact, that some metals dilate more by the 
same degrees of heat than others. Thus, brass 
will dilate twice as much by heat, and con¬ 
sequently contract twice as much by cold, as 
steel. If, then, these differences could be made 
to counteract each other mutually, given points 
at each end of a system of such rods would 
remain stationary the year round, and thus the 
clock would go at the same rate in all climates, 
and during all seasons. 

271. This important object is accomplished 
by the following means : 

Suppose the middle rod, fig. 36, to be made of 
brass, and the two outside ones of steel, all of 
the same length. Let the brass rod be firmly 
fixed to the cross pieces at each end. Let the 
steel rod a , be fixed to the lower cross piece, 


On what depends the number of vibrations which a pendulum makes in a 
given time? What is the medium length of a pendulum beating seconds? 
Why does a common clock go faster in winter than ir. summer. What is 
necessary in respect to the pendulum, to make the clock go true the yea 
round * What is the principle on which the gridiron pendulum is constructed 
6 # 









66 


PENDULUM. 


and />, to the upper crosspiece. The rod a, at its upper end, 
passes through the cross piece, and, in like manner, b passes 
through the lower one. This is done to prevent these small 
rods from playing backwards and forwards, as the pendulum 
swings. 

272. Now, as the middle rod is lengthened by the heat 
twice as much as the outside ones, and the outside rods 
together are twice as long as the middle one, the actual 
length of the pendulum can neither be increased nor dimin¬ 
ished by the variations of temperature. 

273. To make this still plainer, suppose the 
lower cross piece, fig. 37, to be standing on a 
table, so that it could not be lengthened down¬ 
wards, and suppose, by the heat of summer, 
the middle rod of brass should increase one 
inch in length. This would elevate the upper 
cross piece an inch, but at the same time the 
steel rod a, swells half an inch, and the steel 
rod b , half an inch, therefore the two points, c 
and d , would remain exactly at the same dis¬ 
tance from each other. 

274. As it is the force of gravity which draws the weight 
of the pendulum from the highest point of its arc downwards, 
and as this force increases, or diminishes, as bodies approach 
towards the centre of the earth, or recede from it, so the pen¬ 
dulum will vibrate faster, or slower, in proportion as this at¬ 
traction is stronger or weaker. 

275. Now, it is found that the earth at the equator rises 
higher from its centre than it does at the poles, for towards 
the poles it is flattened. The pendulum, therefore, being 
more strongly attracted at the poles than at the equator, vi¬ 
brates faster. For this reason, a clock that would keep ex¬ 
act time at the equator, would gain time at the poles, for the 
rate at which a clock goes, depends on the number of vibra¬ 
tions its pendulum makes. Therefore, pendulums, in order 
to beat seconds, must be shorter at the equator, and longer at 
the poles. 


What are the metals of which this instrument is made ? Explain fig. 36, 
and give the reason why the length of the pendulum will not change by the 
variations of temperature ? Explain fig. 37. What is the downward force 
which makes the pendulum vibrate ? Explain the reason why the same clock 
would go faster at the poles, and slower at the equator. How can a clock 
which goes true at the equator be made to go true at the poles ? Will a clock 
«teep equal time at the foot and on the top of a high mountain ? 










PENDULUM. 


67 


Fig. 


For the same reason, a clock which keeps exact time at 
the foot of a high mountain, would move slower on its top. 

276. Metronome .—There is a short pendulum, used by 
musicians for marking time, which may be made to vibrate 
fast or slow, as occasion requires. This little instrument i 3 
called a metronome , and besides the pendulum, consists of 
several wheels, and a spiral spring, by which the whole is 
moved. This pendulum is only ten or twelve inches long, 
and instead of being suspended by the end, like other pendu¬ 
lums, the rod is prolonged above the point of suspension, and 
there is a ball placed near the upper, as well as at the lower 
extremity. 

277. This arrangement will be 
understood by fig. 38, where a is 
the axis of suspension, b the up¬ 
per ball, and c the lower one. 

Now, when this pendulum vibrates 
from the point o, the upper ball 
constantly retards the motion of 
the lower one, by in part counter¬ 
balancing its weight, and thus pre¬ 
venting its full velocity downards. 

278. Perhaps this will be more 
apparent, by placing the pendu¬ 
lum, fig. 39, for a moment on its 
side, and across a bar, at the 
point of suspension. In this 
position, it will be seen, that 
the little ball would prevent 
the large one from falling with 
its full weight, since, were it 

moved to a certain distance from the point of suspension, it 
would balance the large one so that it would not descend at 
all. It is plain, therefore, that the comparative velocity of 
the large ball will be in proportion as the small one is moved 
to a greater or less distance from the point of suspension. 
The metronome is so constructed, the little ball being made 
to move up and down on the rod at pleasure, that its 
vibrations are made to beat the time of a quick or slow tune, 
as occasion requires. 

By this arrangement, the instrument is made to vibrate every 
two seconds, or every half, or quarter of a second, at pleasure. 

Why will it not ? What is the metronome ? How does this pendulum dif¬ 
fer from common pendulums ? How does the upper ball retard the motion of 
the lower one ? How is the metronome made to go faster or slower, at pleasure ? 



Fig. 39. 


-O 


o- 







68 


MECHANICS. 


MECHANICS. 

279. Mechanics is a science which investigates the laws ana 
effects of force and motion. 

280. The practical object of this science is, to teach the 
best modes of overcoming resistances by means of mechani¬ 
cal powers, and to apply motion to useful purposes, by means 
of machinery. 

281. A machine is any instrument by which power, motion, 
or velocity, is applied or regulated. 

282. A machine may be very simple, or exceedingly com¬ 
plex. Thus, a pin is a machine for fastening clothes, and a 
steam engine is a machine for propelling mills and boats. 

As machines are constructed for a vast variety of purposes, 
their forms, powers, and kinds of movement, must depend on 
their intended uses. 

283. Several considerations ought to precede the actual 
construction of a new or untried machine ; for if it does not 
answer the purpose intended, it is commonly a total loss to 
the builder. 

Many a man, on attempting to apply an old principle to a 
new purpose, or to invent a new machine for an old purpose, 
has been sorely disappointed, having found, when too late, 
that his time and money had been thrown away, for want of 
proper reflection, or requisite knowledge. 

284. If a man, for instance, thinks of constructing a ma¬ 
chine for raising a ship, he ought to take into consideration 
the inertia , or weight , to be moved—the force to be applied— 
the strength of the materials, and the space , or situation, he 
has to work in. For, if the force applied, or the strength of 
the materials, be insufficient, his machine is obviously use¬ 
less ; and if the force and strength be ample, but the space 
be wanting, the same result must follow. 

285. If he intends his machine for twisting the fibres of 
flexible substances into threads, he may find no difficulty in 
respect to power, strength of materials, or space to work in, 
but if the velocity, direction , and kind of motion he obtains, 
be not applicable to the work intended, he still loses his 
labor. 


What is mechanics ? What is the object of this science 1 What is a ma 
chine T Mention one of the most simple, and one of the most complex of ma 
chines. 



MECHANICS. 


69 


286 . Thousands of machines have been constructed, 
which, so far as regarded the skill of the workmen, the in¬ 
genuity of the contriver, and the construction of the individ 
ual parts, were models of art and beauty; and, so far as 
could be seen without trial, admirably adapted to the in¬ 
tended purpose. But on putting them to actual use, it has 
too often been found, that their only imperfection consisted 
in a stubborn refusal to do any part of the work intended. 

287. Now, a thorough knowledge of the laws of motion, 
and the principles of mechanics, would, in many instances 
at least, have prevented all this loss of labor and money, and 
spared him so much vexation and chagrin, by showing the 
projector that his machine would not answer the intended 
purpose. 

The importance of this kind of knowledge is therefore ob¬ 
vious, and it is hoped will become more so as we proceed. 

288. Definitions .—In mechanics, as well as in other sci¬ 
ences, there are words which must be explained, either be¬ 
cause they are common words used in a peculiar sense, or 
because they are terms of art, not in common use. All tech¬ 
nical terms will be as much as possible avoided, but still 
there are a few, which it is necessary here to explain. 

289. Force is the means by which bodies are set in mo¬ 
tion, kept in motion, and when moving, are brought to rest. 
The force of gunpowder sets the ball in motion, and keeps 
it moving, until the force of resisting the air, and the force of 
gravity, bring it to rest. 

290. Power is the means by which the machine is moved, 
and the force gained. Thus we have horse power, water 
power, and the power of weights. 

291. Weight is the resistance, or the thing to be moved 
by the force of the power. Thus the stone is the weight to 
be moved by the force of the lever or bar. 

292. Fulcrum , or prop, is the point on which a thing is 
supported, and about which it has more or less motion. In 
raising a stone, the thing on which the lever rests, is the 
fulcrum. 

In mechanics, there are a few simple machines called the 
mechanical powers , and however mixed, or complex, a com¬ 
bination of machinery may be, it consists only of these few 
individual powers. 


What is meant by force in mechanics ? What is meant by power? What 
is understood by weight ? What is the fulcrum ? Are the mechanical poweis 
numerous, or only few in number? 



70 


LEVER. 


W e shall not here burthen the memory of the pupil with 
the names of these powers, of the nature of which he is at 
present supposed to know nothing, but shall explain the ac¬ 
tion and use of each in its turn, and then sum up the whole 
for his accommodation. 

THE LEVER. 

293. Any rod , or bar , which is used in raising a weight , or 
surmounting a resistance , by being placed on a fulcrum , or 
prop, becomes a lever. 

This machine is the most simple of all the mechanical 
powers, and is therefore in universal use. 

294. Fig. 40 re- Fig. 40. 

presents a straight 
lever, or handspike , 
called also a crow¬ 
bar , which is com¬ 
monly used in rais¬ 
ing and moving 
stone and other 
heavy bodies. The 
block b is the weight , 
or resistance, a is the lever , and c, the fulcrum. 

295. The power is the hand, or weight of a man, applied 
at a, to depress that end of the lever, and thus to raise the 
weight. 

It will be observed, that by this arrangement the applica¬ 
tion of a small power may be used to overcome a great re¬ 
sistance. 

296. The force to be obtained by the lever, depends on its 
length, together with the power applied, and the distance of 
the weight and power from the fulcrum. 

Suppose, fig. 41, that a is 
the lever, b the fulcrum, d 
the weight to be raised, and 
c the power. Let d be con¬ 
sidered three times as heavy 
as c, and the fulcrum three 
times as far from c as it is 
from d; then the weight and 
power will exactly balance 
each other. Thus, if the 




What is a lever ? What is the simplest of all mechanical powers ? Ex 
plain fig. 40. Which is the weight ? Where is the fulcrum ? 











LEVER 


71 


bar be four feet long, and the fulcrum three feet from the 
end, then three pounds on the long arm will weigh just as 
much as nine pounds on the short arm, and these proportions 
will be found the same in all cases. 

297. When two weights balance each other, the fulcrum 
is always at the centre of gravity between them, and there¬ 
fore, to make a small weight raise a large one, the fulcrum 
must be placed as near as possible to the large one, since the 
greater the distance from the fulcrum the small weight or 
power is placed, the greater will be its force. 

298. Suppose the weight Fig. 42. 

h , 

pounds, 
fulcrum 



6i 


fig. 42, to be sixteen 
and suppose the 
to be placed so 
near it, as to be raised 
by the power a, of four 
pounds, hanging equally 
distant from the fulcrum 
and the end of the lever. 

If now the power a be removed, and another of two pounds, 
c, be placed at the end of the lever, its force will be just equal 
to a, placed at the middle of the lever. 

299. But let the fulcrum be moved along to the middle of 
"the lever, with the weight of sixteen pounds still suspended 

to it, it would then take another weight of sixteen pounds, 
instead of two pounds, to balance it, fig. 43. 

Thus, the power which Fig. 43. 

would balance 16 pounds, 
when the fulcrum is in one 
place, must be exchanged 
for another power weighing 
eight times as much, when 
the fulcrum is in another 
place. 

300. From these investi¬ 

gations, we may draw the following general truth, or prop¬ 
osition, concerning the lever: “ That the force of the lever 

increases in proportion to the distance of the power from the 




Where is the power applied ? What is the power in this case ? On what 
does the force to be obtained by the lever depend ? Suppose a lever four feet 
long, and the fulcrum one foot from the end, what number of pounds will bal¬ 
ance each other at the ends? When weights balance each other, at what 
point between them must the fulcrum be ? Suppose a weight of 16 pounds on 
the short arm of a lever is counterbalanced by 4 pounds in the middle of the 
'ong arm, what power would balance this weight at the end of the lever ? 







72 


LEVER. 


fulcrum , and diminishes in proportion as the distance of the 
•weight from the fulcrum increases. ” 

301. From this proposition may be drawn the following 
rule, by which the exact proportions between the weight 
or resistance, and the power, may be found. Multiply the 
weight by its distance from the fulcrum; then multiply the 
power by its distance from the same point , and if the products 
are equal, the weight and the power will balance each other. 

302. Suppose a weight of 100 pounds on the short arm 
of a lever, 8 inches from the fulcrum, then another weight, 
or power, of 8 pounds, would be equal to this, at the dis¬ 
tance of 100 inches from the fulcrum; because 8 multiplied 
by 100 is equal to 800 ; and 100 multiplied by 8 is equal to 
800, and thus they would mutually counteract each other. 

303. Many instru¬ 
ments in common use Fig. 44. 

are on the principle of 
this kind of lever. Scis¬ 
sors, fig. 44, consist of 



two levers, the rivet 
being the fulcrum for 
both. The fingers are 
the power, and the 
cloth to be cut, the re¬ 
sistance to be overcome. 

Pincers , forceps , and sugar cutters , are examples of this 
kind of lever. 

304. A common scale-beam, used for weighing, is a lever 
suspended at the centre of gravity, so that the two arms bal¬ 
ance each other. Hence the machine is called a balance. 
The fulcrum, or what is called the pivot , is sharpened, like a 
wedge, and made of hardened steel, so as much as possible 
to avoid friction. 


305. A dish is suspended 
by cords to each end or arm 
of the lever, for the purpose 
of holding the articles to be 
weighed. When the whole 
is suspended at the point a , 
fig. 45, the beam or lever 


Fig. 45. 



Suppose the fulcrum to be moved to the middle of the lever, what power would 
ww pOU u nds? i \ s th( r general proposition drawn from 

hese results . What is the rule for finding the proportions between the weight 
and power ? Give an illustration of this rule. 








IJIVtfR. 


73 


ought to remain in a horizontal position, one of its ends being 
exactly as high as the other. If the weights in the two 
dishes are equal, and the support exactly in the centre, they 
will always hang as represented in the figure. 

306. A very slight variation of the point of support to 
wards one end of the lever, will make a difference in the 
weights employed to balance each other. In weighing a 
pound of sugar, with a scale-beam of eight inches long, if 
the point of support is half an inch too near the weight, the 
buyer would be cheated nearly one ounce, and consequently 
nearly one pound in every sixteen pounds. This fraud might 
instantly be detected by changing the places of the sugar 
and weight, for then the difference would be quite material, 
since the sugar would then seem to want twice as much 
additional weight as it did really want. 

307. The steel-yard differs from the balance, in having 
its support near one end, instead of in the middle, and also 
in having the weights suspended by hooks, instead of being 
placed in a dish. 

308. If we suppose the 

beam to be 7 inches long, 
and the hook, c, fig. 46, to be 
one inch from the end, then 
the pound weight a, will re¬ 
quire an additional pound at 
b , for every inch it is moved 
from it. This, however, sup¬ 
poses that the bar will balance itself, before any weights are 
attached to it. . 

In the kind of lever described, the weight to be raised is 
on one side of the fulcrum, and the power on the other. 
Thus the fulcrum is between the power and the weight. 

309. There is an¬ 
other kind of lever, 
in the use of which, 
the weight is placed 
between the fulcrum 
and the hand. In other 
words, the weight to 
be lifted, and the pow¬ 
er by which ’it is 
moved, are on the 
same side of the prop. 

What instruments operate on the principle of this lerer * Whan the *ois 
sors are used, what is the resistance, and what the power ? 

7 


Fig. 47. 

P 



Fig. 46. 

9 2 £ S 4 5 6 











74 


LEVER. 


310. This arrangement is represented by fig. 47, where 
w is the weight, l the lever, / the fulcrum, and p a pulley, 
over which a string is thrown, and a small weight suspend¬ 
ed, as the power. In the common use of a lever of the 
first kind, the force is gained by bearing down the long 
arm of the lever, which is called prying. In the seconrl 
kind, the force is gained by carrying the long arm in a con¬ 
trary direction, or upward, and this is called lifting. 

311. Levers of the second kind are not so common as the 
first, but are frequently used for certain purposes. The oars 
of a boat are examples of the second kind. The water 
against which the blade of the oar pushes, is the fulcrum, the 
boat is the weight to be moved, and the hands of the man, 
the power. 

312. Two men carrying a load between them on a pole, 
is also an example of this kind of lever. Each man acts as 
the power in moving the weight, and at the same time each 
becomes the fulcrum in respect to the other. 

If the weight happens to slide on the pole, the man to¬ 
wards whom it goes has to bear more of it in proportion as 
its distance from him is less than before. 

313. A load at a, fig. 48, 
is borne equally by the two 
men, being equally distant 
from each other; but at b> 
three quarters of its weight 
would be on the man at that 
end, because three quarters 
of the length of the lever 
would be on the side of the 
other man. 

314. In the third, and 
last kind of lever, the 
weight is placed at one 
end, the fulcrum at the oth¬ 
er end, and the power be¬ 
tween them, or the hand 
is between the fulcrum 
and the weight to be lifted. 



tY,I n J h f scale-beam, where is the fulcrum ? In what position ought 

the scale-beam to hang ? How may a fraudulent scale-beam be made ? How 
?T Cted? uHow does the steel-yard differ from the balance? 
in the first kind of lever, where is the fulcrum, in respect to theweieht and 
power Lithe second kind, where is iie fulcrum, in respect to the weight and 

second' kind called f* 6 the , ^ rs * ca L e <f ? Wat is the aSoSJ 

second kind called? Give examples of the second kind of lever. 









LEVER. 


75 


315. This is represented by fig. 49, where c is the ful¬ 
crum, a the power, suspended over the pulley b , and d is the 
weight to be raised. 

316. This kind of lever works to great disadvantage, since 
the power must be greater than the weight. It is therefore 
seldom used, except in cases where velocity and not force is 
required. In raising a ladder from the ground to the roof of 
a house, men are obliged sometimes to make use of this prin¬ 
ciple, and the great difficulty of doing it, illustrates the me¬ 
chanical disadvantage of this kind of lever. 

317. We have now described three kinds of levers, and, 
we hope, have made the manner in which each kind acts 
plain, by illustrations. But to make the difference between 
them still more obvious, and to avoid all confusion, we will 
here compare them together. 

318. In the first kind, the weight, or resistance, is on the 
short arm of the lever, the power, or hand, on the long arm, 
and the fulcrum between them. In the second kind, the 
weight is between the fulcrum and the hand, or power; and 
in the third kind the hand is between the fulcrum and the 
weight. 


Fig. 50. 




nrsi Kiiiu ui icvci : in , 

what direction do they a«t in the third kind f 












76 


LEVER. 


319. In fig. 50, the weight and hand both set downwards 
In 51, the weight and hand act in contrary directions, the 
hand upwards and the weight downwards, the weight being 
between them. In 52, the hand and weight also act in con¬ 
trary directions, but the hand is between the fulcrum and the 
weight. 

320. Compound Lever. —When several simple levers are 
connected together, and act one upon the other, the machine 
is called a compound lever . In this machine, as each lever 
acts as an individual, and with a force equal to the action of 
the next lever upon it, the force is increased or diminished, 
and becomes greater or less, in proportion to the number or 
kind of levers employed. 

We will illustrate this kind of lever by a single example, 
but must refer the inquisitive student to more extended works 
for a full investigation of the subject. 

Fig. 53. 

_ € V d/ _ 

a ' A 



Fig. 53, represents a compound lever, consisting of 3 sim¬ 
ple levers of the first kind. 

321. In calculating the force of this lever, the rule applies 
which has already been given for the simple lever, namely: 
The length of the long arm is to be multiplied by the moving 
power, and that of the short one , by the weight , or resistance. 
Let us suppose, then, that the three levers in the figure are 
of the same length, the long arms being six inches, and the 
short ones two inches long; required, the weight which a 
moving power of 1 pound at a will balance at b. In the first 
place, 1 pound at a, would balance 3 pounds at e, for the lev¬ 
er being 6 inches, and the power 1 pound, 6x1=6, and the 
short one being 2 inches, 2x3=6. The long arm of the 
second lever being also 6 inches, and moved with a power 
of 3 pounds, multiply the 3 by 6=18; and multiply the 
length of the short arm, being 2 inches, by 9 = 18. These 

What is a compound lever ? By what rule is the force of the compound lev¬ 
er calculated? How many pounds weight will be raised by three levers con¬ 
nected, of eight inches each, with the fulcrum two inches from the end, by a 
power of one pound* 









WHEEL AND AXLE. 


77 

two products being equal, the power upon the long arm of 
the third lever, at d, would be 9 pounds. 9 pounds x 6=54, 
and *27 x 2, is 54; so that one pound at a would balance 27 at h. 

The increase of force is thus slow, because the proportion 
between the long and short arms is only as 2 to 6, or in the 
proportions of 1, 3, 9. 

322. Now suppose the long arms of these levers to be 18 
inches, and the short ones 1 inch, and the result will be sur¬ 
prisingly different, for then 1 pound at a would balance 18 
pounds at e, and the second lever would have a power of 18 
pounds. This being multiplied by the length of the lever, 
18 x 18 = 324 pounds at d. The third lever would thus be 
moved by a power of 324 pounds, which, multiplied by 18 
inches for the weight it would raise, would give 5832 pounds. 

The compound lever is employed in the construction of 
weighing machines, and particularly in cases where great 
weights are to be determined, in situations where other ma¬ 
chines would be inconvenient, on account of their occupying 
too much space. 


WHEEL AND AXLE. 

323. The mechanical power, next to the lever in simplicity, 
is the wheel and axle. It is, however, much more complex than 
the lever. It consists of two wheels, one of which is larger 
than the other, but the small one passes through the larger, 
and hence both have a common centre, on which they turn. 

324. The manner in which 
this machine acts will be 
understood by fig. 54. The 
large wheel a, on turning the 
machine, will take up, or 
throw off, as much more 
rope than the small wheel 
or axle b, as its circumfer¬ 
ence is greater. If we sup¬ 
pose the circumference of 
the large wheel to be four 
times that of the small One, 
then it will take up the rope 
four times as fast. And because a is four times as large as 


Fig. 54. 



If the long arms of the levers be 18 inches, and the short ones one inch, how 
much will a power of one pound balance ? In what machines is the com¬ 
pound lever employed ? What advantages do these machines possess over oth¬ 
ers ? What is the next simple mechanical power to the lever ? Describe this 
machine ? Explain fig. 54. On what principle does this machine act T 









78 


WHEEL AND AXLE. 


b , l pound at d will balance 4 pounds at c, on the opposite 
side. 

325. The principle of this machine is that of the lever, as 
will be apparent by an examination of fig. 55. 

This figure represents the machine Fl s* 55 * 

endwise, so as to show in what man¬ 
ner that lever operates. The two 
weights hanging in opposition to 
each other, the one on the wheel at 
<z, and the other on the axle at 6, act 
in the same manner as if they were 
connected by the horizontal lever a 
b , passing from one to the other, hav¬ 
ing the common centre, c, as a ful¬ 
crum between them. 

326. The wheel and axle, there¬ 
fore, acts like a constant succession 

of levers, the long arm being half the diameter of the wheel, 
and the short one half the diameter of the axle; the common 
centre of both being the fulcrum. The wheel and axle has, 
therefore, been called the perpetual lever. 

327. The great advantage of this mechanical arrange¬ 
ment is, that while a lever of the same power can raise a 
weight but a few inches at a time, and then only in a cer¬ 
tain direction, this machine exerts a continual force, and in 
any direction wanted. To change the direction, it is only 
necessary that the rope by which the weight is to be raised, 
should be carried 



in a line perpen¬ 
dicular to the axis 
of the machine, to 
the place below 
which the weight 
lies, and there be let 
fall over a pulley. 

328. Suppose the 
wheel and axle, 
fig. 56, is erected 
in the third story 
of a store house, 
with the axle over 
the scuttles, or 


Fig. 56. 



In fig. 55, which is the fulcrum, and which the two arms of the lever T What 
s this machine called, in reference to the principle on which it aots? 










WHEEL AND AXLE. 


79 

doors through the floors, so that goods can be raised by it 
from the ground floor, in the direction of the weight a. Sup¬ 
pose, also, that the same store stands on a wharf, where 
ships come up to its side, and goods are to be removed from 
the vessels into the upper stories. Instead of removing the 
goods into the store, and hoisting them in the direction of a, 
it is only necessary to carry the rope 6, over the pulley c, 
which is at the end of a strong beam projecting out from the 
side of the store, and then the goods will be raised in the di¬ 
rection of d , thus saving the labor of moving them twice. 

The wheel and axle, under different forms, is applied to a 
variety of common purposes. 

329. The capstan , in univer¬ 
sal use, on board of ships and 
other vessels, is an axle placed 
upright, with a head, or drum, 

«, fig. 57, pierced with holes 
for the levers b , c, d. The 
weight is drawn by the rope e, 
passing two or three times round 
the axle to prevent its slipping. 

This is a very powerful and 
convenient machine. When not 
in use, the levers are taken out of their places and laid aside, 
and when great force is required, two or three men can push 
at each lever. 

330. The common F is- 58. 

windlass for drawing 
water is another mod¬ 
ification of the wheel 
and axle. The winch , 
or crank , by which it 
is turned, is moved 
around by the hand, 
and there is no dif¬ 
ference in the princi¬ 
ple, whether a whole 
wheel is turned, or a 
single spoke. The winch, therefore, answers to the wheel, 



Fig. 57. 



What is the great advantage of this machine over the lever and other me¬ 
chanical powers ? Describe fig. 56, and point out the manner in which weights 
can be raised by letting fall a rope over the pulley. What is the capstan? 
Where is it chiefly used ? What are the peculiar advantages of this form of 
the wheel and axle ? In the common windlass, what part answers to the 
wheel ? Explain fig. 58. 

















80 


WHEEL AND AXLE. 


while the rope is taken up, and the weight raised by the axle, 
as already described. 

331. In cases where great weights are to be raised, and it 
is required that the machine should be as small as possible, 
on account of room, the simple wheel and axle, modified as 
represented by fig. 58, is sometimes used. 

332. The axle may be considered in two parts, one of 
which is larger than the other. The rope is attached by 
its two ends, to the ends of the axle, as seen in the figure. 
The weight to be raised is attached to a small pulley, or 
wheel, round which the rope passes. The elevation of the 
weight may be thus described. Upon turning the axle, the 
rope is coiled round the larger part, and at the same time it 
is thrown off the smaller part. At every revolution, there¬ 
fore, a portion of the rope will be drawn up, equal to the cir¬ 
cumference of the thicker part, and at the same time a por¬ 
tion, equal to that of the thinner part, will be let down. 
On the whole, then, one revolution of the machine will 
shorten the rope where the weight is suspended, just as 
much as the difference is between the circumference of the 
two parts. 

333. Now to understand the principle 
on which this machine acts, we must 
refer to fig. 59, where it is obvious that 
the two parts of the rope a and 6, equally 
support the weight d , and that the rope, 
as the machine turns, passes from the 
small part of the axle e to the large part 
h , consequently, the weight does not rise 
in a perpendicular line towards c, the 
centre of both, but in a line between the 
outsides of the large and small parts. 

Let us consider what would be the con¬ 
sequence of changing the rope a to the 
larger prrt of the axle, so as to place 
the weight in a line perpendicular to the 

axis of motion. In this case it is obvious that the machine 
would be in equilibrium, since the weight d would be divided 
between the two sides equally, and the two arms of a lever 


Fig. 59. 



Why 1S * he ro P e shortened, and the weight raised? What is the design of 
fig. 59 r Does the weight rise perpendicular to the axis of motion? Suppose 
the cylinder was, throughout, of the same size, what would be the consequence ? 
On what principle does this machine act? Which are the long and short arms 
of the lever, and where is the fulcrum ? 






WHEEL AND AXLE. 


81 

passing through the centre c, would be of equal length, and 
therefore no advantage would be gained. But in the actual 
arrangement, the weight being sustained equally by the 
large and small parts, there is involved a lever power, the 
long arm of which is equal to half the diameter of the large 
part, while the short arm is equal to half the diameter of the 
small part, the fulcrum being between them. 

334. A varying power , producing a constant force .—If a 
power, varying* under any given conditions be required to 
overcome a resistance which varies according to some other 
given conditio^ the one may be accommodated to the other 
by producing a variation in the leverage, by which one or 
both acts. 

335. This is done in Fig. 60. 

the mechanism of the 
watch, of which a, fig. 

60, is the barrel contain¬ 
ing the power in the form 
of a spiral spring, and b 
the fusee which acts as 
a varying lever, and 

through which motion is conveyed to the hands of the watch. 
Now when the watch is first wound up, the main-spring 
within the barrel is closely coiled, and of course acts with 
much more power than afterwards, when it is partly unrolled; 
hence were no means used to equalize this power, every 
watch would run two or three times as fast, when first wound 
up, as afterwards. We shall see that the fusee is a com¬ 
plete remedy for the varying action of the main-spring. Its 
form is a low cone with its surface cut into a spiral groove, 
to receive the chain, which runs round the barrel. Now 
when the watch is wound up by applying the key to the 
axis of the fusee at c, the main-spring, one end of which is 
attached to the diameter of the barrel, and the other to its 
axis, is closely coiled; but as the action begins on the 
smallest part of the fusee, the leverage is small, and the 
power weak; but as the fusee turns, and the spring uncoils, 
the leverage increases in proportion as the strength of the 
spring becomes weaker, and thus the two forces mutually 
equalize each other, and the watch runs at the same rate 



What is the main-spring of a watch ? Where is it contained ? What is the 
fusee of a watch? What is its form? When does the main-spring act with 
most force? How does the fusee equalize this force? Explain how the 
forces of the spring and fusee mutually equalize each other 
















82 


WHEEL AND AXLE. 


until the chain which connects them has run from the barrel 
to the fusee, when it again requires winding, and the same 
process begins again. 

336. System of Wheels .—As the wheel and axle is only a 
modification of the simple lever, so a system of wheels acting 
on each other, and transmitting the power to the resistance, 
is only another form of the compound lever. 

337. Such a com¬ 
bination is shown in 
fig. 61. The first 
wheel, a, by means 
of the teeth, or cogs, 
around its axle, 
moves the second 
wheel, 6, with a force 
equal to that of a 
lever, the long arm 
of which extends 
from the centre of 
the wheel and axle 
to the circumference 
of the wheel, where 
the power p is sus¬ 
pended, and the short arm from the same centre to the ends 
of the cogs. The dotted line c, passing through the centre 
of the wheel a, shows the position of the lever, as the wheel 
now stands. The centre on which the wheel and axle turn, 
it will be obvious, is the fulcrum of this lever. As the wheel 
turns, the short arm of this lever will act upon the long arm 
of the next lever by means of the teeth on the circumference 
of the wheel Z>, and this again through the teeth on the axle 
of 6, will transmit its force to the circumference of the wheel 
d, and so by the short arm of the third lever to the weight w. 
As the power or small weight falls, therefore, the resistance, 
w , is raised, with the multiplied force of three levers acting 
on each other. 

338. In respect to the force to be gained by such a ma¬ 
chine, suppose the number of teeth on the axle of the wheel 
a to be six times less than the number of those on the cir¬ 
cumference of the wheel 6, then b would only turn round 
once, while a turned six times. And, in like manner, if the 



On what principle does a system of wheels act, as represented in fig. 61 ? 
Explain fig. 61, and show how the power p is transferred by the action of levers 






WHEEL AND AXLE. 


8b 

number of teeth on the circumference of d , be six times greater 
than those on the axle of b , then d would turn once, while b 
turned six times. Thus six revolutions of a would make b 
revolve once, and six revolutions of b would make d revolve 
once. Therefore, a makes thirty-six revolutions while d 
makes only one. 

339. The diameter of the wheel a , being three times the 
diameter of the axle of the wheel d , and its velocity of mo¬ 
tion being 36 to 1,3 times 36 will give the weight which a 
power of 1 pound at p would raise at w. Thus 36 x 3 = 108. 
One pound at p would therefore balance 108 pounds at w. 

340. No machine creates force .—If the student has attend¬ 
ed closely to what has been said on mechanics, he will now 
be prepared to understand, that no machine, however simple 
or complex it may be, can create the least degree of force. 
It is true, that one man with a machine may apply a force 
which a hundred could not exert with their hands, but then 
it would take him a hundred times as long. 

341. Suppose there are twenty blocks of stone to be moved 
a hundred feet; perhaps twenty men, by taking each a 
block, would move them all in a minute. One man, with a 
capstan, we will suppose, may move them all at once, but 
this man, with his lever, would have to make one revolution 
for every foot he drew the whole load towards him, and 
therefore to make one hundred revolutions to perform the 
whole work. It would also take him twenty times as long 
to do it, as it took the twenty men. His task, indeed, would 
be more than twenty times harder than that performed by 
the twenty men, for, in addition to moving the stone, he 
would have the friction of the machinery to overcome, which 
commonly amounts to nearly one third of the force em¬ 
ployed. 

Hence there would be an actual loss of power by the use 
of the capstan, though it might be a convenience for the one 
man to do his work by its means, rather than to call in nine¬ 
teen of his neighbors to assist him. 

342. The same principle holds good in respect to other 
machinery, where the strength of man is employed as the 
power, or prime mover. There is no advantage gained, ex- 


What weight will one pound at p balance at w ? Is there any actual power 
gained by the use of machinery ? Suppose 20 men to move 20 stones to a cer¬ 
tain distance with their hands, and one man moves them back to the same 
dace with a capstan, which performs the most actual labor ? Why T Why, 
men, is machinery a convenience f 



84 


WHEEL AND AXLE. 


cent that of convenience. In the use of the most simple of 
all machines, the lever, and where, at the same time, there 
is the least force lost by friction, there is no actual gam of 
power, for what seems to be gained in force is always lost 
in velocity. Thus, if a lever is of such length to raise 100 
pounds an inch by the power of 1 pound, its long arm must 
pass through a space of 100 inches. Thus, what is gained 
in one way is lost in another. 

343. Any power by which a machine is moved, must be 
equal to the resistance to be overcome, and, in all cases 
where the power descends, there will be a proportion be 
tween the velocity with which it moves downwards, and the 
velocity with which the weight moves upwards. There 
will be no difference in this respect, whether the machine be 
simple or compound, for if its force be increased by increas¬ 
ing the number of levers, or wheels, the velocity of the 
moving power must also be increased, as that of the resist 
ance is diminished. 

344. There being, then, always a proportion, between the 
velocity with which the moving force descends, and that 
with which the weight ascends, whatever this proportion 
may be, it is necessary that the power should have to the 
resistance the same ratio that the velocity of the resistance 
has to the velocity of the power. In Other words, “ The 
■power multiplied by the space through which it moves , in a ver¬ 
tical direction , must be equal to the weight multiplied by the 
space through which it moves in a vertical direction.” 

This law is known under the name of “ the law of virtual 
velocities,” and is considered the golden rule of mechanics. 

345. This principle has already been explained, while 
treating of the lever (296); but that the student should want 
nothing to assist him in clearly comprehending so important 
a law, we will again illustrate it in a different manner. 

346. Suppose the weight of ten pounds to be suspended on 
the short arm of the lever, fig. 62, and that the fulcrum is 
only one inch from the weight; then, if the lever be ten 
inches long, on the other side of the fulcrum, one pound at a 
would rafse, or balance, the ten pounds at b. But in rais¬ 
ing the ten pounds one inch in a vertical direction, the long 


In the use of the lever, what proportion is there between the force of the 
short arm, and the velocity of the long arm ? How is this illustrated ? Is it 
said, that the velocity of the power downwards, must be in proportion to that 
of the weight upwards ? Does it make any difference, in this respect, whether 
the machine be simple or compound ? What is the golden rule of mechanics /f 
Under what name is this law known 1 ? 




PULLEY. 


85 



arm of the lever must fall ten inches Fig. 62. 

in a vertical direction, and therefore the 
velocity of a would be ten times the 
velocity of b. 

347. The application of this law, or 
rule, is apparent. The power is one 
pound, and the space through which it 
falls is ten inches, therefore 10x1 = 10. 

The weight is 10 pounds, and the space 
through which it rises is one inch, therefore 1x10=10. 

348. Thus, the power, multiplied by the space through 
which it moves, is exactly equal to the weight, multiplied by 
the space through which it moves. 


349. Again, suppose 
the lever, fig. 63, to be 
thirty inches long from 
the fulcrum to the point 
where the power p is 
suspended, and that the 
weight w is two inches 
from the fulcrum. If 
the power be 1 pound, 
the weight must be 15 
pounds, to produce equi¬ 
librium, and the power p 
must fall thirty inches, to 
raise the weight ic 2 inch- 


Fig. 63. 


es. Therefore the pow¬ 
er being one pound, and the space 30 inches, 30 x 1 = 30. The 
weight being 15 pounds, and the space 2 inches, 15 x 2=30. 

Thus, the power, multiplied by the space through which 
it falls, and the weight multiplied by the space through 
which it rises, are equal. 

However complex the machine may be, by which the 
force of a descending power is transmitted to the weight to 
be raised, the same rule will apply, as it does to the action 
of the simple lever. 


THE PULLEY. 

350. A pulley consists of a wheel , which is grooved on the 
edge, and which is made to turn on its axis, by a chord passing 
over it. 


Explain fig. 62, and show how the rule is illustrated by that figure. Explain 
fig. 63, and show how the same rule is illustrated by it. What is said of the 
application of this rule to complex machines ? 





86 


PULLEY. 


351. Fig. 64 represents a simple 
pulley , with a single fixed wheel. In 
other forms of the machine, the wheel 
noves up and down, with the weight. 

352. The pulley is arranged 
among the simple mechanical pow¬ 
ers ; but when several are connected, 
the machine is called a system of 
pulleys , or a compound pulley. 

353. One of the most obvious ad¬ 
vantages of the pulley is, its enabling 
men to exert their own power, in pla¬ 
ces where they cannot go themselves, 
a rope and wheel, a man can stand on the deck of a ship, and 
hoist a weight to the topmast. 

By means of two fixed pulleys, a weight may be raised 
upward, while the power moves in a horizontal direction. 
The weight will also rise vertically through the same space 
that the rope is drawn horizontally. 

354. Fig. 65 represents two Fi g- 65 - 

fixed pulleys, as they are 
arranged for such a purpose. 

In the erection of a lofty 
edifice, suppose the upper 
pulley to be suspended to 
some part of the building ; 
then a horse pulling at the 
rope a would raise the 
weight w , vertically, as far 
as he went horizontally. 

355. In the use of the 
wheel of the pulley, there is 
no mechanical advantage, 
except that which arises from removing the friction, and 
diminishing the imperfect flexibility of the rope. 

In the mechanical effects of this machine, the result would 
be the same, did it slide on a smooth surface with the same 
ease that its motion makes the wheel revolve. 

356. The action of the pulley is on a different principle 


What is a pulley ? What is a simple pulley ? What is a system of pulleys, 
or a compound pulley ? What is the most obvious advantage of the pulley T 
How must two fixed pulleys be placed to raise a weight vertically, as far as 
the power goes horizontally ? "What is the advantage of the wheel of the 
pulley ? 



Fig. 64. 



Thus, by means of 








PULLEY. 


87 


r 


from that of the wheel and axle. A system of wheels, as 
already explained, acts on the same 
principle as the compound lever. But Fig. 66. 

the mechanical efficacy of a system of 
pulleys is derived entirely from the di¬ 
vision of the weight among the strings 
employed in suspending it. In the use 
of the single fixed pulley, there can be 
no mechanical advantage, since the 
weight rises as fast as the power de¬ 
scends. This is obvious by fig. 64; 
where it is also apparent that the power 
and weight must be exactly equal, to 
balance each other. 

357. In the single movable pulley, fig. Fig. 67. 

66, the same rope passes from the fixed 
point a, to the power p. It is evident here, 
that the weight is supported equally by 
two parts of the string between which it 
hangs. Therefore, if we call the weight w 
ten pounds, five pounds will be supported 
by one string, and five by the other. The 
power then will support twice its own 
weight, so that a person pulling with a 
force of five pounds at p, will raise ten 
pounds at w. The mechanical force, there¬ 
fore, in respect to the power, is as two to one. 

In this example, it is supposed there are 
only two ropes, each of which bears an 
equal part of the weight. 

358. If the number of ropes be increas¬ 
ed, the weight may be increased with the 
same power ; or the power may be dimin¬ 
ished in proportion as the number of ropes 
is increased. In fig. 67, the number of ropes 
sustaining the weight is four, and therefore 
the weight may be four times as great as 
the power. This principle must be evident 
since it is plain that each rope sustains an 
equal part of the weight. The weight may therefore be 


V J 


How does the action of the pulley differ from that of the wheel and axle t 
Is there any mechanical advantage in the fixed pulley ? What weight at p, 
fig. 66, will balance ten pounds at tv ? Suppose the number of ropes be in 
creased, and the weight increased, must the power be increased also ! 
























PULLEY. 


88 

considered as divided into four parts, and each part sustained 
by one rope. 

359. In fig. 68, there is a system of pulleys represented, 
in which the weight is sixteen times the power. 

The tension of the rope d e, Flg 68 - 

is evidently equal to the pow¬ 
er, p, because it sustains it: d, 
being a movable pulley, must 
sustain a weight equal to twice 
the power; but the weight 
which it sustains, is the ten¬ 
sion of the second rope, d, c. 

Hence the tension of the sec¬ 
ond rope is twice that of the 
first, and, in like manner, the 
tension of the third rope is 
twice that of the second, and 
so on, the weight being equal 
to twice the tension of the last 
rope. 

360. Suppose the weight w 
to be sixteen pounds, then the 
two ropes, 8 and 8, would sus¬ 
tain just 8 pounds each, this 
being the whole weight divi¬ 
ded equally between them. 

The next two ropes, 4 and 4, 
would evidently sustain but 
half this whole weight, because the other half is already 
sustained by a rope fixed at its upper end. The next two 
ropes sustain but half of 4, for the same reason ; and the 
next pair, 1 and f, for the same reason, will sustain only half 
of 2. Lastly, the power p will balance two pounds, because 
it sustains but half this weight, the other half being sustained 
by the same rope, fixed at its upper end. 

361. It is evident, that in this system, each rope and pul¬ 
ley which is added, will double the effect of the whole. 
Thus, by adding another rope and pulley beyond 8, the 
weight w might be 32 pounds, instead of 16, and still be 
balanced by the same power. 



Suppose the weight, fig. 67, to be 32 pounds, what will each rope bear ? 
Explain fig. 68, and show what part of the weight each rope sustains, and why 
1 pound at p will balance 16 pounds at w. Explain the reason why each addi¬ 
tional rope and pulley will double the effect of the whole, or why its weight 
may be double that of all the others, with the same power. 















PULLEY. 


80 


362. In our calculations of the effects of pulleys, we have 
allowed nothing for the weight of the pulleys themselves, or 
for the friction of the ropes. In practice, however, it will 
be found, that nearly one third must be allowed for friction, 
and that the power, therefore, to actually raise the weight, 
must be about one third greater than has been allowed. 

363. The pulley, like other ma chines, obeys the laws of 
virtual velocities, already applied to the lever and wheel. 
Thus, “in a system of pulleys , the ascent of the weight, or re¬ 
sistance, is as much less than the descent of the power, as the 
weight is greater than the power?' If, as in the last example, 
the weight is 16 pounds, and the power 1 pound, the weight 
will rise only one foot while the power descends 16 feet: 

364. In the single fixed pulley, the weight and power are 
equal, and, consequently, the weight rises as fast as the 
power descends. 

365. With such a pulley, a man may raise himself up to 
the mast-head by his own weight. Suppose a rope is thrown 
over a pulley, and a man ties one end of it round his body, 
and takes the other end in his hands ; he may raise himself 
up, because, by pulling with his hands, he has the power of 
throwing more of his weight on that side than on the other, 
and when he does this his body will rise. Thus, although 
the power and the weight are the same individual, still the 
man can change his centre of gravity, so as to make the 
power greater than the weight, or the weight greater than 
the power, and thus can elevate one half of his weight in 
succession. 

WHITE’S PULLEY. 

366. In all the pulleys we have described, there is a great 
defect in consequence of the different velocities at which the 
several wheels turn, and the consequent friction to which 
some of them are subjected. 

367. It is obvious that in a system of pulleys, the first 
wheel, or that over which the cord passes, sustaining the 
power, must turn as many times more than the last wheel, 
or that sustaining the weight, as the weight is greater than 
the power. Thus, some of the wheels turn ten, or twenty 
times, while others turn only once, or twice, and of course 


In compound machines, how much of the power must be allowed for the 
friction? How may a man raise himself up by means of a rope and single 
fixed pulley ? What is a great defect in the common pulley ? What pro- 
portions do the revolutions in the first and last wheels bear to each other • 
What are the consequences of friction in the wheels of the pulley ? 




90 


PULLEY. 


every time a wheel revolves, a length of rope equal to its 
circumference must pass over it. If, then, the system con¬ 
sists of many wheels, the friction not only so retatds the 
motion, as to require a much greater power to raise the same 
weight, but the wheels and the ropes are soon worn out, and 
require to be frequently replaced, often at considerable cost. 

368. Now, allowing the diameter of the wheels to be the 
same, the velocities at which they revolve must be measured 
by the length of rope passing over them, and hence their 
different rates of motion, and unequal friction, mentioned 
above. 

369. It has been an object among me¬ 
chanical philosophers to remedy this de¬ 
fect by inventing a system of pulleys, 
the wheels of which should all revolve 
on their axles in the same time, each 
making the same number of revolutions, 
notwithstanding the different lengths of 
rope passing over them, and thus avoid 
a defect common to those in use. 

370. This object seems to have been 
fully attained by Mr. James White, 
whose irivention is represented by fig. 69, 
and which will be understood by the 
following description : In order that the 
successive wheels should revolve in the 
same time, and their circumferences should 
be just equal to the length of rope passing 
over them, Mr. White made them all of 
different diameters. By this construction, 
although the length of rope passing over 
each was different, yet their revolutions 
are equal, both with respect to time and 
number. 

371. But still, were each wheel separate, 
though the object would in part have 
been attained, yet the friction of many 
wheels placed side by side would have 
left the machine imperfect. To remove this defect, the in¬ 
ventor reduced all the wheels in the same system to one, or 
rather, instead of using many wheels, he cut many grooves 


Fig. 69. 



How are the velocities of the different wheels measured ? In what manner 
is it said that the defect with respect to friction might be remedied 7 Describe 
White’s pulley. 





























INCLINED PLANE. 


91 


in the same block. These grooves, as seen in the figure, arc 
of different diameters, corresponding to the length of rope 
passing over each. 

372. By this arrangement all the friction is avoided, ex¬ 
cept that of a pivot at each end, and the lateral friction of a 
single wheel. A single rope sustains the whole, and as in 
other systems, the weight is as many times the power as 
there are ropes sustaining the lower block. This is consid¬ 
ered the most perfect system of pulleys yet invented. 

THE INCLINED PLANE. 

373. The fourth simple me¬ 
chanical power is the inclined 
plane. 

This power consists of a 
plain, smooth surface, which is 
inclined towards, or from the 
earth. It is represented by fig. 

70, where from a to b is the 
inclined plane ; the line from d 
to a, is its height, and that 
from b to d, its base. 

A board with one end on the ground, and the other end 
resting on a block, becomes an inclined plane. 

374. This machine being both useful and easily con¬ 
structed, is in very general use, especially where heavy 
bodies are to be raised only to a small height. Thus a man, 
by means of an inclined plane, which he can readily con¬ 
struct with a board, or couple of bars, can raise a load into 
his wagon, which ten men could not lift with their hands. 

375. The power required to force a given weight up an in¬ 
clined plane , is in proportion to its height , and the length of its 
base , or , in other words , the force must be in proportion to the 
rapidity of its inclination . 

376. The power, 

/?, fig. 71, pulling a 
weight up the inclin¬ 
ed plane, from c to d, 
only raises it in a 
perpendicular direc¬ 
tion from e to d, by 
acting along the 
whole length of the 

~ What is an inclined plane? On what occasions is this power chiefly used! 
Suppose a man wants to load a barrel of cider into his wagon, how does he 
make an inclined plane for this purpose? 



Fig. 70. 










92 


INCLINED PLANE. 


plane. If the plane be twice as long as it is high, that is, 
if the line from c to d be double the length of that from e to 
d, then one pound at/> will balance two pounds any where be¬ 
tween d*and c. It is evident, by a glance at this figure, that 
were the base, that is, the line from e to c, lengthened, the 
height from e to d being the same, that a less power at p 
would balance an equal weight any where on the inclined 
plane ; and so, on the contrary, were the base made shorter, 
that is, the plane more steep, the power must be increased in 
proportion. 

377. Suppose two in¬ 
clined planes, fig. 72, of 
the same height, with 
bases of different lengths ; 
then the weight and pow¬ 
er will be to each other as 
the length of the planes. 

If the length from a to b 
is two feet, and that from b to c one foot, then two pounds 
at d will balance four pounds at w , and so in this proportion, 
whether the planes be longer or shorter. 

378. The same principle, with respect to the vertical ve¬ 
locities of the weight and powers, applies to the inclined 
plane, in common with the other mechanical powers. 

Suppose the inclined plane, Fig. 73. 

fig. 73, to be two feet from a 
to b , and one foot from c to b , 
then, as we have already seen 
by fig. 71, a power of 1 pound 
at p, would balance a weight 
of two pounds at w. Now, in 
the fall of the power to draw 
up the weight, it is obvious 
that its vertical descent must 
be just twice the vertical as¬ 
cent of the weight; for the 
power must fall down the distance from a to b , to draw the 
weight that distance; but the vertical height to which the 
weight w is raised, is only from ctob. Thus the power, be- 

To roll a given weight up an inclined plane, to what must the force be pro¬ 
portioned ? Explain fig. 71. If the length of the long plane, fig. 72, be double 
that of the short one, what must be the proportion between the power and the 
weight ? What is said of the application of the law of vertical velocities to the 
inclined plane ? Explain fig. 73, and show why the power must fall twice as 
far as the weight rises. 



Fig. 72. 










THE WEDGE. 


93 


ing two pounds, must fall two leet, to raise the weight, four 
pounds, one foot; and thus the power and weight, multiplied 
by the several velocities, are equal. 

379. When the power of an inclined plane is considered 
as a machine, it must therefore be estimated by the propor¬ 
tion which the length bears to the height; the power being 
increased in proportion as the elevation of the plain is diminished. 

Hilly roads may be regarded as inclined planes, and loads 
drawn upon them in carriages, considered in reference to the 
powers which impel them, and subject to all the conditions 
which we have stated, with respect to inclined planes. 

380. The power required to draw a load up a hill, is in 
proportion to the length and elevation of the inclined plane. 
On a road perfectly horizontal, if the power is sufficient to 
overcome the friction, and the resistance of the atmosphere, 
the carriage will move. But if the road rise one foot in fif¬ 
teen, besides these impediments, the moving power will have 
to lift one fifteenth part of the load. 

381. If two roads rise, one at the rate of a foot in fifteen 
feet, and another at the rate of a foot in twenty, then the 
same power that would move a given weight fifteen feet on the 
one, would move it twenty feet on the other, in the same time. 

In the building of roads, therefore, both speed and power 
are very often sacrificed to want of judgment, or ignorance 
of these laws. 

382. A road, as every traveler knows, is often continued 
directly over a hill, when half the power, with the increase 
of speed, on a level road around it, would gain the same dis¬ 
tance in half the time. 

Besides, where is there a section of country in which the 
traveler is not vexed with roads, passing straight over hills, 
when precisely the same distance would carry him around 
them on a level plane. To use a homely, but very pertinent 
illustration, “ the bale of a pot is no longer, when it lies 
down, than when it stands up.” Had this simple fact been 
noticed, and its practical bearing carried into effect by road 
makers, many a high hill would have been shunned for a 
circuit around its base, and many a poor horse, could he 
speak, would thank the wisdom of such an invention. 

THE WEDGE. 

383. The next simple mechanical power is the wedge. 
This instrument may be considered as two inclined planes , pla¬ 
ced base to base. It is much employed for the purpose of 
splitting or dividing solid bodies, such as wood and stone. 


94 


SCREW. 


Fig. 74 represents such a wedge as is usu¬ 
ally employed in cleaving timber. This in¬ 
strument is also used in raising ships, and 
preparing them to launch, and for a variety 
of other purposes. Nails, awls, needles, and 
many cutting instruments, act on the princi¬ 
ple of this machine. 

There is much difficulty in estimating the 
power of the wedge, since this depends on the 
force, or the number of blows given it, togeth¬ 
er with the obliquity of its sides. A wedge of 
great obliquity would require hard blows to 
drive it forward, for the same reason that a 
plane, much inclined, requires much force to 
roll a heavy body up it. But were the obli¬ 
quity of the wedge, and the force of each blow 
given, still it would be difficult to ascertain the exact power 
of the wedge in ordinary cases, for, in the splitting of timber 
and stone, for instance, the divided parts act as levers, and 
thus greatly increase the power of the wedge. Thus, in a 
log of wood, six feet long, when split one half of its length, 
the other half is divided with ease, because the two parts 
act as levers, the lengths of which constantly increase, as the 
cleft extends from the wedge. 

THE SCREW. 


Fig 74 . 



IT 


384. The screw is the fifth and last simple mechanical 
power. It may be considered as a modification of the inclined 
plane , or as a winding wedge. It is an inclined plane run¬ 
ning spirally round a spindle, as Fig- 75 . 

will be seen by fig. 75. Sup¬ 
pose a to be a piece of paper, cut 
into the form of an inclined plane 
and rolled round the piece of 
wood d; its edge would form 
the spiral line, called the thread 
of the screw. 

If the finger be placed be¬ 
tween the two threads of a screw, and the screw be turned 
round once, the finger will be raised upward equal to the 


On what principle does the wedge act ? In what case is this power useful ? 
What common instruments act on the principle of the wedge ? What difficul¬ 
ty is there in estimating the power of the wedge ? On what principle does the 
screw act ? How is it shown that the screw is a modification of the inclined 
plane? 










SCREW. 


95 

distance of the two threads apart. In this manner, the fin¬ 
ger is raised up the inclined plane, as it runs round the cyl¬ 
inder. 

385. The power of the screw is 
transmitted and employed by 
means of another screw called the 
nut , through which it passes. 

This has a spiral groove running 
through it, which exactly fits the 
thread of the screw. 

If the nut is fixed, the screw 
itself, on turning it round, advan¬ 
ces forward ; but if the screw is 
fixed, the nut, when turned, ad¬ 
vances along the screw. 

Fig. 76 represents the first kind 
of screw, being such as is common¬ 
ly used in pressing paper, and other substances. The nut, n, 
through which the screw passes, answers also for one of the 
beams of the press. If the screw be turned to the right, it 
will advance downwards, while the nut stands still. 

386. A screw of the second 
kind is represented by fig. 77. 

In this, the screw is fixed, while 
the nut, 7i, by being turned by 
the lever, Z, from right to left, 
will advance down the screw. 

In practice, the screw is nev¬ 
er used as a simple mechanical 
machine ; the power being al¬ 
ways applied by means of a 
lever, passing through the head 
of the screw, as in fig. 76, or 
into the nut, as in fig. 77. 

The screw, therefore, acts with 
the combined power of the inclined plane and the lever, and its 
force is such as to be limited only by the strength of the mate¬ 
rials of which it is made. 

387. In investigating the effects of this machine, we must, 
therefore, take into account both these simple mechanical 

Explain R%'76. Which is the screw, and which the nut? Which way must 
the screw be turned, to make it advance through the nut ? How does the screw, 
fig. 76, differ from fig. 77? Is the screw ever used as a simple machine ? By 
what other simple power is it moved ? What two simple mechanical powers 
are concerned in the force of the screw ? 


Fig. 77. 
S 



Fig. 76. 






















SCREW. 


99 

lowers, so that the screw now becomes really a compound 
engine. 

388. In the inclined plane, we have already seen, that the 
less it is inclined, the more easy is the ascent up it. In ap¬ 
plying the same principle to the screw, it is obvious, that the 
greater the distance of the threads from each other, the more 
rapid the inclination, and, consequently, the greater must be 
the power to turn it, under a given weight. On the contra¬ 
ry, if the thread inclines downwards but slightly, it will turn 
with less power, for the same reason that a man can roll a 
heavy weight up a plane but little inclined. Therefore, the 
finer the screw, or the nearer the threads to each other, the 
greater will be the pressure under a given power. 

389. Let us suppose two screws, the one having the 
threads one inch apart, and the other half an inch apart; 
then the force which the first screw will give with the same 
power at the lever will be only half that given by the second. 
The second screw must be turned twice as many times 
round as the first, to go through the same space, but what is 
lost in velocity is gained in power. At the lever of the first, 
two men would raise a given weight to a given height by 
making one revolution; while at the lever of the second, one 
man would raise the same weight to the same height, by 
making two revolutions. 

390. It is apparent that the length of the inclined plain, 
up which a body moves in one revolution, is the circumfer¬ 
ence of the screw, and its height the interval between the 
threads. The proportion of its power would therefore be 
“ as the circumference of the screw , to the distance between the 
threads , so is the weight to the power .” 

391. By this rule the power of the screw alone can be 
found; but as this machine is moved by means of the lever, 
we must estimate its force by the combined power of both. 
In this case) the circumference described by the end of the 
lever employed, is taken, instead of the circumference of the 
screw itself. The means by which the force of the screw 
may be found, is therefore by multiplying the circumference 
which the lever describes by the power. Thus, “ the power 
multiplied by the circumference which it describes , is equal to 

Why does the nearness of the threads make a difference in the force of the 
screw ? Suppose one screw, with its threads one inch apart, and another half 
an inch apart, what will be their difference in force ? What is the length of 
the inclined plane, up which a body moves by one revolution of the screw ? 
What would be the height to which the same body would move at one revolu¬ 
tion ? How is the force of the screw estimated ? How may the efficacy of the 
crew be increased ? 



SCREW. 


97 


the weight or resistance , multiplied by the distance between the 
two contiguous threads .” Hence the efficacy of the screw 
may he increased, by increasing the length of the lever by 
which it is turned, or by diminishing the distance between 
the threads. If, then, we know the length of the lever, the 
distance between the threads, and the weight to be raised, 
we can readily calculate the power; or, the power being 
given, and the distance of the threads and the length of the 
lever known,, we can estimate the weight the screw will 
raise. 

392. Thus, suppose the length of the lever to be forty 
inches, the distance of the threads one inch, and the weight 
8000 pounds; required, the power, at the end of the lever, to 
raise the weight. 

393. The lever being 40 inches, the diameter of the circle, 
which the end describes, is 80 inches. The circumference 
is a little more than three times the diameter, but we will call 
it just three times. Then, 80x3=240 inches, the circum¬ 
ference of the circle. The distance of the threads is 1 inch, 
and the weight 8000 pounds. To find the power, multiply 
the weight by the distance of the threads, and divide by the 
circumference of the circle. Thus, 


circum. in. weight. power 

240 x 1 : : 8000 = 33£ 

The power at the end of the lever must therefore be 33* 
pounds. In practice this power would require to be increased 
about one-third, on account of friction. 

394. Perpetual Screw. —The force of the screw is some¬ 
times employed to turn a wheel, by acting on its teeth. In 
this case it is called th e perpetual screw. 

395. Fig. 78 represents Fi s- 78 - 

such a machine. It is appa¬ 
rent, that by turning the 
crank c, the wheel will re¬ 
volve, for the thread of the 
screw passes between the 
cogs of the wheel. By means 
of an axle, through the centre 
of this wheel, like the com¬ 
mon wheel and axle, this be¬ 
comes an exceedingly power¬ 
ful machine, but like all other 
contrivances for obtaining 
great power, its effective mo- 

9 










SCREW. 


9 £ 

tion is exceedingly slow. It has, however, some disadvan 
tages, and particularly the great friction between the thread 
of the screw and the teeth of the wheel, which prevents it 
from being generally employed to raise weights. 

396. All these mechanical powers resolved into three.- -We 
have now enumerated and described all the mechanical 
powers usually denominated simple. They are six in num¬ 
ber, namely, the Lever, Wheel and Axle, Pulley, Wedge, 
Inclined Plane, and Screw. 

397. In respect to the principle on which they act, they 
may be resolved into three simple powers, namely, the lever, 
the inclined plane, and the pulley; for it has been shown 
that the wheel and axle is only another fonn of the lever, and 
that the screw is but a modification of the inclined plane. 

It is surprising, indeed, that these simple powers can be so 
arranged and modified, as to produce the different actions in 
all that vast variety of intricate machinery which men have 
invented and constructed. 

398. The variety of motions we witness in the little engine 
which makes cards, by being supplied with wire for the teeth, 
and strips of leather to stick them through, would itself seem 
to involve more mechanical powers than those enumerated. 
This engine takes the wire from a reel, bends it into the form 
of teeth; cuts it off; makes two holes in the leather for the 
tooth to pass through ; sticks it through; then gives it an¬ 
other bend, on the opposite side of the leather; graduates the 
spaces between the rows of teeth, and between one tooth and 
another; and, at the same time, carries the leather back¬ 
wards and forwards, before the point where the teeth are in¬ 
troduced, with a motion so exactly‘corresponding with the 
motions of the parts which make and stick the teeth, as not 
to produce the difference of a hair’s breadth in the distance 
between them. 

399. All this is done without the aid of human hands, any 
farther than to put the leather in its place, and turn a crank; 
or, in some instances, many of these machines are turned at 
once, by means of three or four dogs, walking on an inclined 
plane which revolves. 


The length of the lever, the distance between the threads, and the weight 
being known, how can the power be found ? Give an example. What is the 
screw called when it is employed to turn a wheel? What is the object of this 
machine for raising weights ? How many simple mechanical powers are there, 
and what are they called? How can they be resolved into three simple pow¬ 
ers? What is said of the card-making machine ? Wh^t are the chief mechan¬ 
ical powers concerned in its motions ? 



SCREW. 


90 


400. Such a machine displays the wonderful ingenuity 
and perseverance of man, and at first sight would seem to 
set at nought the idea that the lever and wheel were the 
chief simple powers concerned in its motions. But when 
tnese motions are examined singly and deliberately, we are 
soon convinced that the wheel, variously modified, is the 
principal mechanical power in the whole engine. 

401. Use of Machinery .—It has already been stated, (332) 
that notwithstanding the vast deal of time and ingenuity 
which men have spent on the construction of machinery, and 
in attempting to multiply their powers, there has, as yet, 
been none produced, in which the power was not obtained 
at the expense of velocity, or velocity at the expense of 
power; and, therefore, no actual force is ever generated by 
machinery. 

402. Suppose a man able to raise a weight by means of a 
compound pulley of ten ropes, which it would take ten men 
to raise by one rope without pulleys. If the weight is to 
be raised a yard, the ten men by pulling their rope a yard 
will do the work. But the man with the pulleys must draw 
his rope ten yards to raise the weight one yard, and in ad¬ 
dition to this, he has to overcome the friction of the ten pul¬ 
leys, making about one-third more actual labor than was 
employed by the ten men. But notwithstanding these in¬ 
conveniences, the use of machinery is of vast importance to 
the world. 

403. On board of a ship, a few men will raise an anchor 
with a capstan, which it would take ten or twenty times the 
same number to raise without it, and thus the expense of 
shipping men expressly for this purpose is saved. 

404. One man with a lever, may move a stone which it 
would take twenty men to move without it, and though it 
should take him twenty times as long, he would still be the 
gainer, since it would be more convenient, and less expensive 
for him to do the work himself, than to employ twenty others 
to do it for him. 

405. When men employ the natural elements as a power 
to overcome resistance by means of machinery, there is a 
vast saving of animal labor. Thus mills, and all kinds of 
engines, which are kept in motion by the power of water, or 


Is there any actual force generated by machinery? Can great velocity and 
great force be produced by the same machinery ? Why not ? Which performs 
the greatest labor, ten men who lift a weight with their hands, or one man who 
does the same with ten pulleys ? Why ? 



SCREW. 


100 

wind, or steam, save animal labor equal to the power it takes 
to keep them in motion. 

406. Five Mechanical Powers in one Machine .—An engi¬ 
neer, it is said, for the purpose of drawing a ship out of the 
water to be repaired, combined the mechanical powers repre¬ 
sented by fig. 79, and perhaps no machine ever eonstrueted 
gives greater force with so small a power. 



It involves the lever «, wheel and axle 6, the pulley c and 
d, the inclined plane d, and the screw e. 

407. To estimate the force of this engine it is necessary to 
know the length of the lever, diameter of the wheel, &c. 

Suppose then, the sizes of the different powers are as fol¬ 
lows, viz: 

Length of the lever a, . ,.18 inches. 

Distance of the threads e,.1 inch. 

Diameter of the wheel b ,.4 feet. 

Diameter of the axle,.1 foot. 

Pulleys c and d, d fixed,.4 strings. 

Height of the plane d one-half its length, . 2 

Suppose the man turns the lever a with the power equal 
to 100 pounds, the force on the ship would thus be found, for 
the different laws and rules referring to each mechanical 
power. 

1. One hundred pounds on the lever a , would 

become a force by means of the screw on the 
wheel b of. 

2. Diameter of wheel four times that of the axle, 


3. The number of pulley strings,. 


4. Height of the inclined plane half its length, 


Pounds. 

11,309.76 

4 

45,239.04 

4 

180,956.16 

2 

361,912.32 














HYDROSTATICS. 


10. 

The force on the ship therefore would be equal to 361,912 
pounds, o_ about 161 tons. 


HYDROSTATICS. 

408. Hydrostatics is the science which treats of the weight , 
pressure, and equilibrium of water, or other fluids when in a 
state of rest. 

409. Hydraulics is that j5krt of the science of fluids which 
treats of water in motion, and the means of raising and con¬ 
ducting it in pipes, or otherwise, for all sorts of purposes. 

410. The subject of water at rest, will first claim investi¬ 
gation, since the laws which regulate its motion will be best 
understood by first comprehending those which regulate its 
pressure. 

411. A fluid is a substance whose particles are easily moved 
among each other, as air and water. 

412. The air is called an elastic fluid, because it is easily 
compressed into a smaller bulk, and returns again to its origi¬ 
nal state when the pressure is removed. Water is called a 
won-elastic fluid, because it admits of little diminution of bulk 
under pressure. 

413. The non-elastic fluids are perhaps more properly 
called liquids , but both terms are employed to signify vi ater 
and other bodies possessing its mechanical properties. The 
term fluid, when applied to the air, has the word elastic be¬ 
fore it. 

414. One of the most obvious properties of fluids, is the 
facility with which they yield to the impressions of other 
bodies, and the rapidity with which they recover their form¬ 
er state, when the pressure is removed. The cause of this, 
is apparently the freedom with which their particles slide 
over, or among each other; their cohesive attraction being 
so slight as to be overcome by the least impression. On this 
want of cohesion among their particles seems to depend the 
peculiar mechanical properties of these bodies. 


What are the five mechanical powers employed in fig. 79 ? Point out on the 
cut the place of each power. What is hydrostatics ? How does hydraulics 
differ from hydrostatics ? W T hat is a fluid ? What is an elastic fluid ? Why is 
air called an elastic fluid ? What substances are called liquids ? What is 
one of the most obvious properties of liquids ? On what do the peculiar me 
chanical properties of fluids depend ? 

9* 




102 


HYDROSTATICS. 


415. In solids, there is such a connection between the par¬ 
ticles, that if one part moves, the other part must move also 
But in fluids, one portion of the mass may be in motion, 
while the other is at rest. In solids, the pressure is always 
downwards, or towards the centre of the earth’s gravity; 
but in fluids, the particles seem to act on each other as 
wedges, and hence, when confined, the pressure is sideways, 
and even upwards, as well as downwards. 

416. Water has commonly been called a non¬ 
elastic substance, but it is found that under great 
pressure its volume is diminished, and hence it is 
proved to be elastic. The most decisive experi¬ 
ments on this subject were made within a few 
years by Mr. Perkins. 

417. These experiments were made by means 
of a hollow cylinder, fig. 80, which was closed at 
the bottom, and made water tight at the top, by 
a cap, screwed on. Through this cap, at a, 
passed the rod Z>, which was five-sixteenths of an 
inch in diameter. The rod was so nicely fitted 
to the cap, as also to be water tight. Around the 
rod at c, there was placed a flexible ring, which 
could be easily pushed up or down, but fitted so closely as 
to remain on any part where it was placed. 

418. A cannon of sufficient size to receive this cylinder, 
which was three inches in diameter, was furnished with a 
strcng cap and forcing pump, and set vertically into the 
ground. The cannon and cylinder were next filled with 
water, and the cylinder, with its rod drawn out, and the ring 
placed down to the cap, as in the figure, was plunged into 
the cannon. The water in the cannon was then subjected 
to an immense pressure by means of the forcing pump, after 
which, on examination of the apparatus, it was found that 
the ring c, instead of being where it was placed, was eight 
inches up the rod. The water in the cylinder being com¬ 
pressed into a smaller space, by the pressure of that in the 
cannon, the rod was driven in, while under pressure, but was 
forced out again by the expansion of the water, when the 
pressure was removed. Thus, the ring on the rod would 
indicate the distance to which it had been forced in, during 
the greatest pressure 


Fig. 80 

b 


In what respect does the pressure of a fluid differ from that of a solid ? Is 
water an elastic, or a non-elastic fluid ? Describe fig. 80, and show how water 
was found to be elastic T 










HYDROSTATICS. 


103 


•119. This experiment proved that water, under the pres- 
1 r r nnn ° ne thousanc1 atmospheres, that is, the weight of 
lo,0u0 pounds to the square inch, was reduced in bulk about 
one part in 24. 

So slight a degree of elasticity under such immense pres¬ 
sure, is not appreciable under ordinary circumstances, and 
therefore in practice, or in common experiments on this fluid 
water is considered as non-elastic. 

EQUAL PRESSURE OF WATER. 

420. The particles of water, and other fluids, when confined, 
press on the vessel which confines them, in all directions, both 
upwards, downwards, and sideways. 

From this property of fluids, together with their weight 
or gravity, very unexpected and surprising effects are pro¬ 
duced. 

421. The effect of this property, which we shall first ex¬ 
amine, is, that a quantity of water, however small, will bal¬ 
ance another quantity, however large. Such a proposition 
at first thought might seem very improbable. But on exam¬ 
ination, we shall find that an experiment with a very simple 
apparatus will convince any one of its truth. Indeed, we 
every day see this principle established by actual experiment, 
as will be seen directly. 

422. Fig. 81 represents a common 
coffee-pot, supposed to be filled up to 
the dotted line a , with a decoction of 
coffee, or any other liquid. The coffee, 
we know, stands exactly at the same 
height, both in the body of the pot, and 
in its spout. Therefore, the small 
quantity in the spout , balances the large 
quantity in the pot, or presses with the 
same force downwards, as that in the body of the pot presses 
upwards. This is obviously true, otherwise, the large quan¬ 
tity would sink below the dotted line, while that in the spout 
would rise above it, and run over. 

423. The same principle is more strikingly illustrated by 
fig. 82. 


Fig. 81. 



In what proportion does the bulk of water diminish under a pressure of 
15,000 pounds to the square inch? In common experiments,is water consid¬ 
ered elastic, or non-elastic ? When water is confined, in what direction does 
it press ? How does the experiment with the coffee-pot show that a small 
quantity of liquid will balance a large one ? 



104 


HYDROSTATICS. 


Suppose the cistern a to be capable 
of holding one hundred gallons, and 
into its bottom there be fitted the tube 
b , bent, as seen in the figure, and capa¬ 
ble of containing one gallon. The 
top of the cistern, and that of the tube, 
being open, pour water into the tube 
at c, and it will rise up through the 
perpendicular bend into the cistern, 
and if the process be continued, the 
cistern will be filled by pouring water 
into the tube. Now it is plain, that 
the gallon of water in the tube presses against the hun¬ 
dred gallons in the cistern, with a force equal to the pressure 
of the hundred gallons, otherwise, that in the tube would be 
forced upwards higher than that in the cistern, whereas, we 
find that the surfaces of both stand exactly at the same height. 

424. From these experiments we learn, “ that the pressure 
of a fluid is not in proportion to its quantity, but to its height, 
and that a large quantity of water in an open vessel, presses 
down with no more force than a small quantity of the same 
height 

425. Pressure equal in vessels of all sizes and shapes .— 
The size or shape of a vessel is of no consequence, for if a 
number of vessels, dififering entirely from each other in fig¬ 
ure, position, and capacity, have a communication made 
between them, and one be filled with water, the surface of 
the fluid, in all, will be at exactly the same elevation. If, 

^therefore, the water stands at an equal height in all, the 
pressure in one must be just equal to that in another, and so 
equal to that in all the others. 

Fig. 83. 


Fig. 82. 

C 



6 5 lb S 2 1 CL 



426. To make this obvious, suppose a number of vessels, 
of different shapes and sizes, as represented by fig. 83, to 



























HYDROSTATICS. 


105 


have a communication between them, by means of a small 
tube, passing from the one to the other. If, now, one of 
these vessels be filled with water, or if water be poured into 
the tube a, all the other vessels will be filled at the same in¬ 
stant, up to the line b c. Therefore, the pressure of the 
water in a , balances that in 1,2, 3, &c., while the pressure 
in each of these vessels is equal to that in the other, and so 
an equilibrium is produced throughout the whole series. 

427. If an ounce of water be poured into the tube a , it will 

produce a pressure on the contents of all the other vessels, 
equal to the pressure of all the others on the tube; for, it 
will force the water in all the other vessels to rise upwards 
to an equal height with that in the tube itself. Hence, we 
must conclude, that the pressure in each vessel is not only 
equal to that in any of the others, but also that the pressure 
in any one is equal to that in all the others. ^ 

428. From this we learn, that the shape or size of a ves¬ 
sel has no influence on the pressure of its liquid contents, 
but that the pressure of water is as its height, whether the 
quantity be great or small. We learn, also, that in no case 
will the weight of a quantity of liquid, however large, force 
another quantity, however small, above the level of its own 
surface. 

429. This is proved by other experiments ; for if, from a 
pond situated on a mountain, water be conveyed in an inch 
tube to the valley, a hundred feet below, the water will rise 
just a hundred feet in the tube ; that is, exactly to the level 
of the surface of the pond. Thus the water in the pond, 
and that in the tube, press equally against each other, and 
produce an exact equilibrium. 

Thus far we have considered the fluid as acting only in 
vessels with open mouths, and therefore at liberty to seek 
its balance, or equilibrium, by its own gravity. Its pressure, 
we have seen, is in proportion to its height, and not to it? 
bulk. 

430. Now, by other experiments, it is ascertained, that the 


Explain fig. 82, and show how the pressure in the tube is equal to the pres¬ 
sure in the cistern. What conclusion, or general truth, is to be drawn from 
these experiments ? What difference does the shape or size of a vessel make 
in respect to the pressure of a fluid on its bottom ? Explain fig. 83, and show 
how the equilibrium is produced. Suppose an ounce of water be poured into 
the tube a , what will be its effect on the contents of the other vessels ? What 
conclusion is to be drawn from pouring the ounce of water into the tube a ? 
What is the reason that a large quantity of water will not force a small quan¬ 
tity above its own level ? Is the force of water in proportion to its height, or 
its quantity ? 



10G 


HYDROSTATICS. 


Fig. 84. 


Q* 


piessure of a liquid is in proportion to its height , and the area 
of its base. 

Suppose a vessel, ten feet high, 
and two feet in diameter, such as is 
represented at a, fig. 84, to be filled 
with water; there would be a certain 
amount of pressure, at c, near the 
bottom. Let d represent another ves¬ 
sel, of the same diameter at the bot¬ 
tom, but only a foot high, and closed 
at the top. Now if a small tube, the 
fourth of an inch in diameter, be in¬ 
serted into the cover of the vessel d ) 
and this tube be carried to the height 
of the vessel a, and then the vessel 
^and tube be filled with water, the 
pressure on the bottoms and sides of 
both vessels to the same height will 
be equal, and jets of water starting from d , and c, will have 
exactly the same force, and rise to the same height. 

431. This might at first seem improbable, but to convince 
ourselves of its truth, we have only to consider, that any im¬ 
pression made on one portion of the confined fluid in the 
vessel d, is instantly communicated to the whole mass. 
Therefore, the water in the ^ tube b presses with the same 
force on every other portion of the water in d , as it does on 
that small portion over which it stands. 

This principle is illustrated in a very strik¬ 
ing manner, by the experiment, which has 
often been made, of bursting the strongest 
wine cask with a few ounces of water. 

432. Suppose a, fig. 85, to be such a cask, 
already filled with water, and suppose the 
tube 6, thirty feet high, to be screwed, water 
tight, into its head. When water is poured 
into the tube, so as to fill it gradually, the 
cask will show increasing signs of pressure, 
by emitting the water through the pores of 
the wood, and between the joints; and, 
finally, as the tube is filled, the cask will 
burst asunder. 

433. The same apparatus will serve to 
illustrate the upward pressure of water; for, 
if a small stop-cock be fitted to the upper 
head, on turning this, when the tube is filled, 


Fig. 85. 











PRESSURE OF WATER. 


107 


ft jet of water will spirt up with a force, and to a height, 
that will astonish all who never before saw such an experi¬ 
ment. 

In theory, the water will spout to the same height with 
that which gives the pressure, but, in practice, it is found to 
fall short, in the following proportions : 

434. If the tube be twenty feet high, and the orifice for 
the jet half an inch in diameter, the water will spout nearly 
nineteen feet. If the tube be fifty feet high, the jet will rise 
upwards of forty feet, and if a hundred feet, it will rise above 
eighty feet. It is understood, in every case, that the tubes 
are to be kept full of water. 

The height of these jets show the astonishing effects that 
a small quantity of fluid produces when pressing from a 
perpendicular elevation. 

435. Hydrostatic Bellows .—An instrument called the hy¬ 
drostatic bellows, also shows, in a striking manner, the great 
force of a small quantity of water, pressing in a perpendic¬ 
ular direction. 

436. This instrument consists, of two boards, connected 

together with strong leather, in the manner of the common 
bellows. It is then furnished with Fi s- 86 - 

a tube a, fig. 86, which communi¬ 
cates between the two boards. A 
person standing on the upper board 
may raise himself up by pouring 
water into the tube. If the tube a> 
holds an ounce of water, and has 
an area equal to a thousandth part 
of the area of the top of the bellows, 
one ounce of water in the tube will 
balance a thousand ounces placed 
on the bellows. 

437. Hydraulic Press .—This prop¬ 
erty of water was applied by Mr. Bra¬ 
mah to the construction of his hy¬ 
draulic press. But instead of a high 

tube of water, which in most cases could not be so readily 



How is a small quantity of water shown to press equal to a large quantity 
by fig 84 ? Explain the reason why the pressure is as great at d, as at c. 
How is the same principle illustrated by fig. 85? How is the upward pressure 
of water illustrated by the same apparatus ? Under the pressure of a column 
of water twenty feet high, what will be the height of the jet ? Under a 
pressure of a hundred feet, how high will it rise ? What is the hydrostatic 
bellows? What property of water is this instrument designed to show ? 







jyg PRESSURE OF WATER. 

obtained, he substituted a strong forcing pump, and instead 
of the leather bellows, a metallic pump barrel and piston. 

438. This arrangement Fig. 87. 

will be understood by fig. 

87, where the pump barrel, 
a , b, is represented as divi¬ 
ded lengthwise, in order to 
show the inside. The pis¬ 
ton, c, is fitted so accurate¬ 
ly to the barrel, as to work 
up and down water tight; 
both barrel and piston be¬ 
ing made of iron. The 
thing to be broken, or press¬ 
ed, is laid on the flat surface, i, there being above this, a 
strong frame to meet the pressure, not shown in the figure. 
The small forcing pump, of which d is the piston, and A, the 
lever by which it is worked, is also made of iron. 

439. Now, suppose the space between the small piston and 
the large one, at 10 , to be filled with water, then, on forcing 
aown the small piston, d, there will be a pressure against 
the large piston, c, the whole force of which will be in pro¬ 
portion as the aperture in which c works, is greater than 
that in which d works. If the piston, d , is half an inch in 
diameter, and the piston, c, one foot in diameter, then the 
pressure on c will be 576 times greater than that on d. 
Therefore, if we suppose the pressure of the small piston to 
be one ton, the large piston will be forced up against any 
resistance, with a pressure equal to the weight of 576 tons. 
It would be easy for a single man to give the pressure of a 
ton at d 7 by means of the lever, and, therefore, a man, with 
this engine, would be able to exert a force equal to the 
weight of near 600 tons. 

440. It is evident that the force to be obtained by this 
principle, can only be limited by the strength of the materi¬ 
als of which the engine is made. Thus, if a pressure of two 
tons be given to a piston, the diameter of which is only a 
quarter of an inch, the force transmitted to the other piston, 
if three feet in diameter, would be upwards of 40,000 tons; 
but such a force is much too great for the strength of any 
material with which we are acquainted. 



Explain fig. 87. Where is the piston? Which is the pump barrel, in which 
it works ? In the hydrostatic press, what is the proportion between the pres 
ure given by the small piston, and the force exerted on the large one l 















HYDROSTATICS. 


J09 


441. A small quantity of water, extending to a great ele¬ 
vation, would give the pressure above described, it being 
only for the sake of convenience, that the forcing pump is 
employed instead of a column of water. 

442. Rupture of a Mountain.^- There is no doubt, but in the 
operations of nature, great effects are sometimes produced 
among mountains, by a small quantity of water finding its 
way to a reservoir in the crevices of the rocks far beneath. 


Fig. 88. 



443. Suppose, in the interior of a mountain, at a , fig. 88, 
there should be a space of ten yards square, and an inch 
deep, filled with water, and closed up on all sides; and sup¬ 
pose that, in the coursemf time, a small fissure, no more than 
an inch in diameter, snould be opened by the water, from the 
height of two hundred feet above, down to this little reser¬ 
voir. The consequence might be, that the side of the moun¬ 
tain would burst asunder, for the pressure, under the circum¬ 
stances supposed, would be equal to the weight of five thou¬ 
sand tons. 

444. Pressure on vessels with oblique sides .—It is obvious, 
that in a vessel, the sides of which are every where perpen¬ 
dicular to each other, that the pressure on the bottom will be 
as the height, and that the pressure on the sides will every 
where be equal at an equal depth of the liquid. 

445. But it is not so obvious, that in a vessel having 
oblique sides, that is, diverging outwards from the bottom, 
or converging from the bottom towards the top, in what 
manner the pressure will be sustained. 

What is the estimated force which a man could give by one of these engines ? 
If the pressure of two tons be made on a piston of a quarter of an inch i' 
diameter, what will be the force transmitted to the other piston of three feet in 
diameter ? What is said of the pressure of water in the crevices of mountains 
and its effects ? 


10 








HYDROSTATICS. 


110 

446. Now, the pressure on the bottom of any vessel, no 
matter what the shape may f>e, is equal to the height of the 
fluid, and the area of the bottom. 

447. Hence the pres¬ 
sure on the bottom of 
the vessel sloping out¬ 
wards, fig. 89, will be 
just equal to what it 
would be, were the sides 
perpendicular, and the 
same would be the case did the sides slope inwards instead 
of outwards. 

448. In a vessel of this shape, the sides sustain a pressure 
equal to the perpendicular height of the fluid, above any giv¬ 
en point. Thus, if the point 1 sustain a pressure of one 
pound, 2, being twice as far below the surface, will have a 
pressure equal to two pounds, and so in this proportion with 
respect to the other eight parts marked on the side of the 
vessel. 

449. On the contrary, did the sides of the vessel slope in¬ 
wards instead of outwards, as 
represented by fig, 90, still the 
same consequences would en¬ 
sue, that is, the perpendicu¬ 
lar height, in both cases, would 
make the pressure equal. For 
although, in the latter case, 
the perpendicular height is not 
above the point pressed upon, still the same effect is produced 
by the pressure of the fluid in the direction perpendicular to 
the plane of the side, and since fluids press equally in all di¬ 
rections, this pressure is just the same as though it were per¬ 
pendicularly above the point pressed upon, as in the direction 
of the dotted lines. 

450. To show that this is the case, we will suppose that 
P, fig. 87, is a particle of the liquid at the same depth below 
the surface as the division marked 5 on the side of the ves¬ 
sel ; this particle is evidently pressed downwards by the in¬ 
cumbent weight of the column of fluid P, a. But since fluids 
press equally in all directions, this particle must be pressed 


What is the pressure on the bottom of a vessel containing a fluid equal 
.o ? Suppose the sides of the vessel slope outwards, what effect does this 
produce on the pressure ? , 









WATER LEVEL. 


Ill 

upwards and sideways with the same force that it is pressed 
downwards, and, therefore, must be pressed from P towards 
the side of the vessel, marked 5, with the same force that it 
would be if the pressure was perpendicular above that part of 
the vessel. 

451. From all that has been stated, we learn, that if the 
sides of the vessels, 86 and 87, be equally inclined, though 
in contrary directions to their bottoms, and the vessels be 
filled with equal depths of water, the sides being of equal di¬ 
mensions, will be pressed equally, though the actual quantity 
of fluid in each, be quite different from each other. 

WATER LEVEL. 

452. We have seen, that in whatever situation water is 

placed, it always tends to seek a level Thus, if several ves¬ 
sels communicating with each other be filled with water, the 
fluid will be at the same height in all, and the level will be 
indicated by a straight line drawn through all the vessels, as 
in fig. 80. ’ 

It is on the principle of this tendency, that the little instru¬ 
ment called the water level is constructed. 

453. The form of this 
instrument is represent¬ 
ed by fig. 91. It con¬ 
sists of a tube, a, b, with 
its two ends turned at 
right-angles, and left 
open. Into one of the 
ends is poured water or mercury, until the fluid rises a little 
in the angles of the tube. On the surface of the fluid, at each 
end, are then placed small floats, carrying upright frames, 
across which are drawn small wires or hairs, as seen at c 
and d. These hairs are called the sights , and are across the 
line of the tube. 

454. It is obvious that this instrument will always indi¬ 
cate a level, when the floats are at the same height, in re¬ 
spect to each other, and not in respect to their comparative 
heights in the ends of the tube, for if one end of the instru¬ 
ment be held lower than the other, still the floats must al- 


Fig. 91. 



How is it shown that the pressure of the fluid at 5, is equal to what it would 
have been had the liquid been perpendicular above that point ? On what prin¬ 
ciple is the water-level constructed ? Describe the manner in which the level 
with sights is used, and the reason why the floats will always be at the same 
height ? 







] 12 


WATER LEVEL 


ways be at the same height. To use this level, therefore, we 
have only to bring the two sights, so that one will range with 
the other; and on placing the eye at c, and looking towards 
<Z, this is determined in a moment. 

This level is indispensable in the construction of canals 
and aqueducts, since the engineer depends entirely on it, to 
ascertain whether the water can be carried over a given hill 
or mountain. 

455. The common spirit level 
consists of a glass tube, fig. 92, 
filled with spirit of wine, excepting 
a small space in which there is left 
a bubble of air. This bubble, when 
the instrument is laid on a level surface, will be exactly in 
the middle of the tube, and therefore to adjust a level, it is 
only necessary to bring the bubble to this position. 

The glass tube is inclosed in a brass case, which is cut 
out on the upper side, so that the bubble may be seen, as 
represented in the figure. 

456. This instrument is employed by builders to level 
their work, and is highly convenient for that purpose, since 
it is only necessary to lay it on a beam to try its level. 

457. Improved Water Level .—In this edition we add the 

figure and description of a more complete water level than 
that seen at fig. 92. • 


Fig. 92. 
A 


^Bggglgjl^ m 


458. Let A, fig. 
93, be a straight 
glass tube, having 
two legs, or two 
other glass tubes, 
rising from each 
end at right-angles. 
Let the tube A, 
and a part of the 
legs, be filled with 
mercury or some 
other liquid, and 
on the surfaces, a 
b , of the liquid, let 
floats be placed 
carrying upright 
wires, to the ends 


Fig. 93 

2 . 1 




3 



What is the use of the level ? Describe the common spirit level, and th« 
method of using it. 



















SPECIFIC GRAVITY. 


113 


of which are attached sights at 1,2. These sights are rep 
resented bj 3, 4, and consist of two fine threads, or hairs, 
stretched at right-angles across a square, and are placed at 
right-angles to the length of the instrument. 

459. They are so adjusted that the point where the hairs 
intersect each other, shall be at equal heights above the 
floats. This adjustment may be made in the following 
manner: 

460. Let the eye be placed behind one of the sights, look¬ 
ing through it at the other, so as to make the points, where 
the hairs intersect, cover each other, and let some distant 
object, covered by this point, be observed. Then let the in¬ 
strument be reversed, and let the points of intersection of the 
hairs be viewed in the same way, so as to cover each other. 
If they are observed to cover the same distant object as be¬ 
fore, they will be of equal heights above the surfaces of the 
liquid. But, if the same distant points be not observed in 
the direction of these points, then one or the other of the 
sights must be raised or lowered, by an adjustment provided 
for that purpose, until the points of intersection be brought 
to correspond. These points will then be properly adjusted, 
and the line passing through them will be exactly horizontal. 
All points seen in the direction of the sights will be on the 
level of the instrument. 

461. The principles on which this adjustment depends are 
easily explained: if the intersections of the hairs be at the 
same distance from the floats, the line joining those intersec¬ 
tions will evidently be parallel to the lines joining the sur¬ 
faces a, b , of the liquid, and will therefore be level. But if 
one of these points be more distant from the floats than the 
other, the line joining the intersections will point upwards 
if viewed from the lower sight, and downwards if viewed 
from the higher one. 

462. The accuracy of the results of this instrument, will 
be greatly increased by lengthening the tube A. 

SPECIFIC GRAVITY 

463. If a tumbler be filled with water to the brim, and an 
egg, or any other heavy solid, be dropped into it, a quantity of 
the fluid, exactly equal to the size of the egg, or other solid, 
will be displaced , and will flow over the side of the vessel. 
Bodies which sink in water, therefore, displace a quantity 
of the fluid equal to their own bulks. 

Explain by fig. 93, how an exact line may be obtained by adjusting the 

10 * 



SPECIFIC GRAVITY. 


Fig. 94. 



114 

464. Now, it is found by experiment, that when any 
solid substance sinks in water, it loses, while in the fluid, a 
portion of its weight, just equal to the weight of the bulk of 
water which it displaces. This is readily made evident by 
experiment. 

465. Take a piece of 
ivory, or any other sub¬ 
stance that will sink in 
water, and weigh it ac¬ 
curately in the usual 
manner; then suspend it 
by a thread, or hair, in 
the empty cup e, fig. 94, 
and then balance it, as 
shown in the figure.— 

Now pour water into the 
cup, and it will be found 
that the suspended body will lose a part of its weight, so that 
a certain number of grains must be taken from the opposite 
scale, in order to make the scales balance as before the water 
was poured in. The number of grains taken from the oppo¬ 
site scale, show the weight of a quantity of water equal to 
the bulk of the body so suspended. 

466. It is on the principle, that bodies weigh less in the 
water than they do when weighed out of it, or in the air, 
that water becomes the means of ascertaining their specific 
gravities, for it is by comparing the weight of a body in the 
water, with what it weighs out of it, that its specific gravity 
is determined. 

467. Thus, suppose a cubic inch of gold weighs 19 ounces, 
and on being weighed in water, weighs only 18 ounces, or 
loses a nineteenth part of its weight, it will prove that gold, 
bulk for bulk, is nineteen times heavier than water, and thus 
19 would be the specific gravity of gold. And so if a cube 
of copper weigh 9 ounces in the air, and only 8 ounces in 
the water, then copper, bulk for bulk, is 9 times as heavy as 
water, and therefore has a specific gravity of 9. 

468. If the body weighs less, bulk for bulk, than water, 
it is obvious that it will not sink in it, and therefore weights 
must be added to the lighter body, to ascertain how much 
less it weighs than water. 


When a solid is weighed in water, w r hy does it lose a part of its weight 1 
How much less will a cubic inch of any substance weigh in water than in air 1 
How is proved by fig. 94, that a body weighs less in water than in air* 














SPECIFIC GRAVITY. 


115 

The specific gravity of a body, then, is merely its weight, 
compared with the same bulk of water; and water is thus 
made the standard by which the weights of all other bodies 
are compared. 

469. How to take the Specific Gravity. —To take the spe¬ 
cific gravity of a solid which sinks in water, first weigh the 
body in the usual manner, and note down the number of 
grains it weighs. Then, with a hair, or fine thread, suspend 
it from the bottom of the scale-dish, in a vessel of water, as 
represented by fig. 94. As it weighs less in water, weights 
must be added to the side of the scale where the body is sus¬ 
pended, until they exactly balance each other. Next, note 
down the number of grains so added, and they will show the 
difference between the weight of the body in air, and in water. 

It^s obvious, that the greater the specific gravity.of the 
body, the less, comparatively, will be this difference, because 
each body displaces only its own bulk of water, and some 
bodies of the same bulk will weigh many times as much as 
others. 

470. For example, we will suppose that a piece of platina, 
weighing 22 ounces, will displace an ounce of water, while 
a piece of silver, weighing 22 ounces, will displace two 
ounces of water. The platina, therefore, when suspended as 
above described, will require one ounce to make the scales 
balance, while the same weight of silver will require two 
ounces for the same purpose. The platina, therefore, bulk 
for bulk, will weigh twice as much as the silver, and will 
have twice as much specific gravity. 

Having noted down the difference between the weight of 
the body in air and in water, as above explained, the specific 
gravity is found by dividing the weight in air, by the loss in 
water. The greater the loss, therefore, the less will be the 
specific gravity, the bulk being the same. 

Thus, in the above example, 22 ounces of platina was 
supposed to lose one ounce in water, while 22 ounces of sil¬ 
ver lost two ounces in water. Now 22, divided by 1, the 
loss of the platina, is 22 ; and 22 divided by 2, the loss in the 
silver, is 11. So that the specific gravity of platina is 22, 
while that of silver is 11. The specific gravities of these 
metals are, however, a little less than here estimated. [ For 
other methods of taking specific gravity, see Chemistry .] 


What is the specific gravity of a body ? How are the specific gravities of 
solid bodies taken? Why does a heavy body weigh comparatively less in the 
water than a light one ? 



116 


HYDROMETER. 


HYDROMETER. 

471. The hydrometer is an instrument , by which the specific 
gravities of fluids are ascertained , by the depth to which the 
instrument sinks below their surfaces. 

Suppose a cubic inch of lead loses, when weighed in wa¬ 
ter, 253 grains, and when weighed in alcohol, only 209 grains, 
then, according to the principle already recited, a cubic inch 
of water actually weighs 253, and a cubic inch of alcohol 
209 grains, for when a body is weighed in a fluid, it loses 
just the weight of the fluid it displaces. 

472. Water, as we have already seen, (466,) is the stand¬ 
ard by which the weights of othor bodies are compared, and 
by ascertaining what a given bulk of any substance weighs 
in water, and then what it weighs in any other fluid, the 
comparative weight of water and this fluid will be known. 
For if, as in the above example, a certain bulk of water 
weighs 253 grains, and the same bulk of alcohol only 209 
grains, then alcohol has a specific gravity nearly one fourth 
less than water. 

It is on this principle that the hydrometer is constructed. 
It is composed of a hollow ball of glass, or metal, with a 
graduated scale rising from its upper part, and a weight on 
its under part, which serves to balance it in the fluid. 

Such an instrument is represented by fig. 

95, of which b is the graduated scale, and 
a the weight, the hollow ball being between 
them. 

473. To prepare this instrument for use, 
weights, in grains, or half grains, are put 
into the little ball a, until the scale is car¬ 
ried down, so that a certain mark on it co¬ 
incides exactly with the surface of the wa¬ 
ter. This mark, then, becomes the stand¬ 
ard of comparison between water and any 
other liquid, in which the hydrometer is 
placed. If plunged into a fluid lighter than 
water, it will sink below the mark, and 
consequently the fluid will rise higher on 

Having taken the difference between the weight of body in air and in water, 
by what rule is its specific gravity found? Give the example stated, and show 
how the difference between the specific gravities of platina and silver is ascer¬ 
tained. What is the hydrometer? Suppose a cubic inch of any substance 
weighs 253 grains less in water than in air, what is the actual weight of a cu¬ 
bic inch of water ? On what principle is the hydrometer founded ? How is 
his instrument formed ? How is the hydrometer prepared for use ? 


Fig. 95. 








SYPHON 


117 


the scale. If the fluid is heavier than water, the scale will 
rise above the surface in proportion, and thus it is ascertained 
in a moment, whether any fluid has a greater or less specific 
gravity than water. 

To know precisely how much the fluid varies from the 
standard, the scale is marked off into degrees, which indi¬ 
cate grains by weight, so that it is ascertained, very exactly, 
how much the specific gravity of one fluid differs from that 
of another. 

474. Water being the standard by which the weights of 
other substances are compared, it is placed as the unit, or 
point of comparison, and is therefore 1, 10, 100, or 1000, the 
ciphers being added whenever there are fractional parts ex¬ 
pressing the specific gravity of the body. It is always un¬ 
derstood, therefore, that the specific gravity of water is 1, 
and when it is said a body has a specific gravity of 2, it is 
only meant that such a body is, bulk for bulk, twice as 
heavy as water. If the substance is lighter than water, it 
has a specific gravity of 0, with a fractional part. Thus 
alcohol has a specific gravity of 0,809, that is, 809, water 
being 1000. 

By means of this instrum nt, it can be told with great ac¬ 
curacy, how much water has been added to spirits, for the 
greater the quantity of water, the higher will the scale rise 
above the surface. 

The adulteration of milk with water, can also be readily 
detected with it, for as new milk has a specific gravity of 
1032, water being 1000, a very small quantity of water mix¬ 
ed with it would be indicated by the instrument. 

THE SYPHON. 

475. Take a tube, bent like the letter U, and having filled 
it with water, place a finger on each end, and in this state 
plunge one of the ends into a vessel of water, so that the end 
in the water shall be a little the highest, then remove the fin¬ 
gers, and the liquid will flow out, and continue to do so, un¬ 
til the vessel is exhausted. 

A tube acting in this manner, is called a syphon , and is 
represented by fig. 96. The reason why the water flows 


How is it known by this instrument, whether the fluid is lighter or heavier 
than water? What is the standard by which the weights of other bodies are 
compared ? What is the specific gravity of water ? When it is said that the 
specific gravity of a body is 2, or 4, what meaning is intended to be conveyed? 
Alcohol has a specific gravity of 809; what, in reference to this, is the specific 
gravity of water ? In what manner is a syphon made ? 



SYPHON. 


1 18 

from the end of the tube a , and, 
consequently, ascends through 
the other part, is, that there is a 
greater weight of the fluid from 
h to < 2 , than from c to because 
the perpendicular height from b 
to a is the greatest. The weight 
of the water from b to a falling 
downwards, by its gravity, tends 
to form a vacuum, or void space, 
in that leg of the tube; but the 
pressure of the atmosphere on the 
water in the vessel, constantly forces the fluid up the other 
leg of the tube, to fill the void space, and thus the stream is 
continued as long as any water remains in the vessel. 

476. Intermitting Springs .—The action of the syphon de¬ 
pends upon the same principle as the action of the pump, 
namely, the pressure of the atmosphere, and therefore its ex¬ 
planation properly belongs to Pneumatics. It is introduced 
here merely for the purpose of illustrating the phenomena of 
intermitting springs; a subject which belongs to Hydro¬ 
statics. 

Some springs, situated on the sides of the mountains, flow 
for a while with great violence, and then cease entirely. After 
a time, they begin to flow again, and then suddenly stop, as 
before. These are called intermitting springs. Among ig¬ 
norant and superstitious people, these strange appearances 
have been attributed to witchcraft, or the influence of some 
supernatural power. But an acquaintance with the laws of 
nature will dissipate such ill founded opinions, by showing 
that they owe their peculiarities to nothing more than natural 
syphons, existing in the mountains from whence the water 
flows. 

477. Fig. 97 is the section of a mountain and spring, 
showing how the principle of the syphon operates to produce 
the effect described. Suppose there is a crevice, or hollow 
in the rock from a to 6, and a narrow fissure leading from it, 
in the form of the syphon, b c. The water, from the rills f e, 
filling the hollow, up to the line a d , it will then discharge 
itself through the syphon, and continue to run until the wa- 


Fig. 9(1 



Explain the reason why the water ascends through one leg of the syphon, 
and descends through the other. What is an intermittent spring ? How is the 
phenomenon of the intermittent spring explained ? Explain fig. 97, and show 
the reason why such a spring will flow, and cease to flow, alternately ? 





HYDRAULICS. 


no 

ter is exhausted down to the leg of the syphon b , when it 
will cease. Then the water from the rills continuing to run 
until the hollow is again filled up to the same line, the sy¬ 
phon again begins to act, and again discharges the contents 
of the reservoir as before, and thus the springy, at one moment, 
flows with great violence, and the next moment ceases entirely. 
Fig. 97. 



The hollow, above the line a d , is supposed not to be filled 
with the water at all, since the syphon begins to act when¬ 
ever the fluid rises up to the bend d. 

During the dry seasons of the year, it is obvious, that such 
a spring would cease to flow entirely, and would begin again 
only when the water from the mountain filled the cavity 
through the rills. ^ 

Such springs, although not very common, exist in various 
parts of the world. Dr. Atwell has described one in the 
Philosophical Transactions, which he examined in Devon¬ 
shire, in England. The people in the neighborhood, as 
usual, ascribed its actions to some sort of witchery, and ad¬ 
vised the doctor, in case it did not ebb and flow readily, 
when he and his friend were both present, that one of them 
should retire, and see what the spring would do, when only 
the other was present. 


HYDRAULICS. 

478. It has been stated , (408,) that Hydrostatics is that 
branch of Natural Philosophy , which treats of the weight, 
pressure , and equilibrium of fluids , and that Hydraulics has 
for its object, the investigation of the laws which regulate 
fluids in motion 





120 


HYDRAULICS. 


Tf the pupil has learned the principles on which the pres¬ 
sure and equilibrium of fluids depend, as explained under the 
former article, he will now be prepared to understand the 
laws which govern fluids when in motion. 

The pressure of water downwards, is exactly in the same 
proportion to its height, as is the pressure of solids in the 
same direction. 

479. Suppose a vessel of three inches in diameter has a 
billet of wood set up in it, so as to touch only the bottom, 
and suppose the piece of wood to be three feet long, and to 
weigh nine pounds ; then the pressure on the bottom of the 
vessel will be nine pounds. If another billet of wood be set 
on this, of the same dimensions, it will press on its top with 
the weight of nine pounds, and the pressure at the bottom 
will be eighteen pounds, and if another billet be set on this, 
the pressure at the bottom will be twenty-seven pounds, ami 
so on, in this ratio, to any height the column is carried. 

480. Now the pressure of fluids is exactly in the same 
proportion ; and when confined in pipes, may be considered 
as one short, column set on another, each of which increases the 
pressure of the lowest, in proportion to their number and height 

481. Thus, notwithstanding the lateral 
pressure of fluids, their downward pressure is Fi 
as their height. This fact will be found of 
importance in the investigation of the princi¬ 
ples of certain hydraulic machines, and we 
have, therefore, endeavored to impress it on 3 
the mind of the pupil by fig. 98, where it will 
be seen, that if the pressure of three feet of 
water be equal to nine pounds on the bottom 
of the vessel, the pressure of twelve feet will 6 
be equal to thirty-six pounds. 

482. The quantity of water which will be 

discharged from an orifice of a given size, 
will be in proportion to the height of the col- 9 
umn of water above it, for the discharge will 
increase in velocity in proportion to the pres¬ 
sure, and the pressure, we have already seen, 
will be in a fixed ratio to the height. 12.^ 


ig. 98. 


-18 


-27 




How does the science of Hydrostatics differ from that of Hydraulics ? 
Does the downward pressure of water differ from the downward pressure of 
solids, in proportion ? How is the downward pressure of water illustrated ? 
Without reference to the lateral pressure, in what proportion do fluids press 
downwards ? What will be the proportion of a fluid discharged from an orifice 
of a given size ? 








HYDKAUMCS. 


12*. 


483. If a vessel, fig. 

99, be filled with water, 
and three apertures be 
made in its sides at the 
points a, b , and c, the 
fluid will be thrown out 
in jets, and will fall to¬ 
wards the earth, in the 
curved lines, a, b, and c. 

The reason why these 
curves differ in shape, 
is, that the fluid is acted 
on by two forces, name¬ 
ly, the pressure of the water above the jet, which produces 
its velocity forward, and the action of gravity, which impels 
it downward. It therefore obeys the same laws that solids 
do when projected forward, and falls down in curved lines, 
the shapes of which depend on their relative velocities. 

The quantity of water discharged, being in proportion to 
the pressure, that discharged from each orifice will differ in 
quantity, according to the height of the water above it. 

484. It is found, however, that the velocity with which a 
vessel discharges its contents,, does not depend entirely on 
the pressure, but in part on the kind of orifice through which 
the liquid flows. It might be expected, for instance, that a 
tin vessel of a given capacity, with an orifice of, say an inch 
in diameter through its side, would part with its contents 
sooner than another of the same capacity and orifice, whose 
side was an inch or two thick, since the friction through the 
tin might be considered much less than that presented by 
the other orifice. But it has been found, by experiment, 
that the tin vessel does not part with its contents so soon as 
another vessel, of the same height and size of orifice, from 
which the water flowed through a short pipe. And, on 
varying the length of these pipes, it is found that the most 
rapid discharge, other circumstances being equal, is through 
a pipe, whose length is twice the diameter of its orifice. 
Such an aperture discharged 82 quarts, in the same time 


Fig. 99. 



Why do the lines described by the jets from the vessel, fig. 99, differ in 
shape ? What two forces act upon the fluid as it is discharged, and how do 
these forces produce a curved line ? Does the velocity with wmch a fluid is 
discharged, depend entirely on the pressure ? What circumstance, besides 
pressure, facilitates the discharge of water from an orifice ,f In a tube dis 
charging water with the greatest velocity, what is the proportion between its 
diameter and its length ? 

11 









HYDRAULICS. 


122 

that another vessel of tin, without the pipe, discharged 62 

quarts. _ . 

This surprising difference is accounted for, by supposing 
that the cross currents, made by the rushing of the wate) 
from different directions towards the orifice, mutually inter¬ 
fere with each other, by which the whole is broken, and 
thrown into confusion by the sharp edge of the tin, and 
hence the water issues in the form of spray, or of a screw, 
from such an orifice. A short pipe seems to correct this 
contention among opposing currents, and to smooth the pas¬ 
sage of the whole, and hence we may observe, that from 
such a pipe, the stream is round and well defined. 

485. Proportion between the pressure and the velocity of 
discharge .—If a small orifice be made in the side of a vessel 
filled with any liquid, the liquid will flow out with a force 
and velocity equal to the pressure which the liquid before 
exerted on that portion of the side of the vessel before the 
orifice was made. 

Now, as the pressure of fluids is as their heights, it fol¬ 
lows, as above stated, that if several such orifices are made, 
the lowest will discharge the greatest, while the highest will 
discharge the least quantity of the fluid. 

486. The velocity of discharge, in the several orifices of 
such a vessel, will show a remarkable coincidence between 
the ratio of increase in the quantity of liquid, and the in¬ 
creased velocity of a falling body. 

Thus, if the tall vessel, fig. 100, of 
equal dimensions throughout, be filled 
with the water, and a small orifice be 
made at one inch from the top, or below 
the surface, as at 1 ; and another at 2, 

4 inches below this; another at 9 
inches; a fourth at 16 inches ; and a 
fifth at 25 inches ; then the velocities of 
discharge, from these several orifices, 
will be in proportion of 1, 2, 3, 4, 5. 

To express this more obviously we 
will place the expressions of the several 
velocities in the upper line of the follow¬ 
ing table, the lower numbers, corres¬ 
ponding, expressing the depths of die 
several orifices. 


V elocity, 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Depth, 

1 

4 

9 

16 

25 

b6 

49 

64 

81 

100 


Fig. 100. 
































HYDRAULICS. 


123 


187. Thus it appears, that to produce a twofold velocity 
a fourfold height is necessary. To obtain a threefold ve¬ 
locity of discharge, a ninefold height is required, and for a 
fourfold velocity, sixteen times the height is necessary, and 
so in this proportion, as shown by the table. (See 86.) 

483. To apply this law to the motion of falling bodies, it 
appears that if a body were allowed to fall freely from the 
surface of the water downwards, being unobstructed by the 
fluid, it would, on arriving at each of the orifices, have ve¬ 
locities proportional to those of the water discharged at the 
said orifices respectively. Thus, whatever velocity it would 
have acquired on arriving at 1, the first orifice, it would have 
doubled that velocity on arriving at 2, the second orifice, 
trebled it on arriving at the third orifice, and so on with 
respect to the others. (See 90.) 

489. In order to establish the remarkable fact, that the 
velocity with which a liquid spouts from an orifice in a ves¬ 
sel, is equal to the velocity which a body would acquire in 
falling unobstructed from the surface of the liquid to the 
depth of the orifice, it is only necessary to prove the truth of 
the principle in any one particular case. 

490. Now it is manifestly true, if the orifices be presented 
downwards, and the column of fluid over it be of small 
height, then this indefinitely small column will drop out of 
the orifice by the mere effect of its own weight, and, there¬ 
fore, with the same velocity as any other falling body ; but 
as fluids transmit pressure in all directions, the same effect 
will be produced, whatever may be the direction of the orifice. 
Hence, if this principle be true, then the direction and size of 
the orifice can make no difference in the result, so that the 
principle above explained, follows as an incontrovertible fact. 

FRICTION BETWEEN SOLIDS AND FLUIDS. 

491. The rapidity with which water flows through pipes 
of the same diameter, is found to depend much on the nature 
of their internal surfaces. Thus a lead pipe, with a smooth 
aperture, under the same circumstances, will convey much 


What is the proportion between the quantity of fluid discharged through an 
orifice of tin and through a short pipe? What are the proportions between 
the velocities of discharge and the heights of the orifices, as above explained? 
If in fig. 97, orifices are made at the distance of 1, 4, 9, 16 and 25 inches from 
the top, then in what ratio of velocity will the water be discharged ? How is 
it proved that the velocity of the spouting liquid is equal to that of a falling 
body ? Suppose a lead and a glass tube, of the same diameter, which will de 
iver the greatest quantity of liquid in the same time ? 



124 


HYDRAULICS. 


more water than one of wood, where the surface is rough, 
or beset with points. In pipes, even where the surface is 
as smooth as glass, there is still considerable friction, for m 
all cases, the water is found to pass more rapidly in the 
middle of the stream than it does on the outside, where it 
rubs against the sides of the tube. 

The sudden turns, or angles of a pipe, are also found to 
be a considerable obstacle to the rapid conveyance of the 
water, for such angles throw the fluid into eddies or currents 
by which its velocity is arrested. 

In practice, therefore, sudden turns are generally avoided, 
and where it is necessary that the pipe should change its 
direction, it is done by means of as large a circle as con¬ 
venient. 

Where it is proposed to convey a certain quantity of 
water to a considerable distance in pipes, there will be a 
great disappointment in respect to the quantity actually de¬ 
livered, unless the engineer takes into account the friction, 
and the turnings of the pipes, and makes large allowances 
for these circumstances. If the quantity to be actually 
delivered ought to fill a two-inch pipe, one of three inches 
will not be too great an allowance, if the water is to be con¬ 
veyed to any considerable distance. 

In practice, it will be found that a pipe of two inches in 
diameter, one hundred feet long, will discharge about five 
times as much water as one of one inch in diameter of the 
same length, and under the same pressure. This difference 
is accounted for, by supposing that both tubes retard the 
motion of the fluid, by friction, at equal distances from their 
inner surfaces, and consequently that the effect of this cause 
is much greater in proportion, in a small tube, than in a 
large one. 

492. The effect of friction in retarding the motion of 
fluids is perpetually illustrated in the flowing of rivers and 
brooks. On the side of a river, the water, especially 
where it is shallow, is nearly still, while in the middle of the 
stream it may run at the rate of five or six miles an hour. 
For the same reason, the water at the bottoms of rivers is 
much less rapid than at the surface. This is often proved 


Why will a glass tube deliver most? What is said of the sudden turnings 
of a tube, in retarding the motion of the fluid ? How much~more water will a 
two inch tube of a hundred feet long discharge, than a one-inch tube of the 
same length ? How is this difference accounted for ? How do rivers show 
the effect of friction in retarding the motion of their waters ? 



HYDRAULICS. 


125 


by the oblique position of floating substances, which in still 
water would assume a vertical direction. 

493. Thus, suppose the stick of 
wood e, fig. 101, to be loaded at one 
end with lead, of the same diameter 
as the wood, so as to make it stand 
upright in still water. In the current 
of a river, where the lower end nearly 
reaches the bottom, it will incline as 
in the figure, because the water is 
more rapid towards the surface than 
at the bottom, and hence the tendency 
of the upper end to move faster than 
the lower one, gives it an inclination forward. 

MACHINES FOR RAISING WATER. 

494. The common pump, though a hydraulic machine, 
depends on the pressure of the atmosphere for its effect, and 
therefore its explanation comes properly under the article 
Pneumatics, where the consequences of atmospheric pressure 
will be illustrated. 

Such machines only as raise water without the assistance 
of the atmosphere, come properly under the present article. 

495. Archimedes' Screw .—Among these, one of the most 
curious, as well as ancient machines, is the screw of Archi- 
medes , and which was invented by that celebrated philoso¬ 
pher, two hundred years before the Christian era, and then 
employed for raising water, and draining land in Egypt. 
















126 


HYDRAULICS 


496. It consists of a large tube, fig. 102, coiled round a 
shaft of wood to keep it in place, and give it support. Both 
ends of the tube are open, the lower one being dipped into 
the water to be raised, and the upper one discharging it in 
an intermitting stream. The shaft turns on a support at 
each end, that at the upper end being seen at a, the lower 
one being hid by the water. As the machine now stands, 
the lower bend of the screw is filled with water, since it is 
below the surface c, d. On turning it by the handle, from 
left to right, that part of the screw now filled with water will 
rise above the surface c, d , and the water having no place 
to escape, falls into the next lowest part of the screw at e. 
At the next revolution, that portion which, during the last, 
was at e , will be elevated to g, for the lowest bend will re¬ 
ceive another supply, which in the mean time will be trans¬ 
ferred to e , and thus, by a continuance of this motion, the 
water is finally elevated to the discharging orifice p. 

This principle is readily illustrated by winding a piece of 
lead tube round a walking stick, and then turning the whole 
with one end in a dish of water, as shown in the figure. 

497. Theory of Archimedes' Screw .—By the following cuts 
and explanations, the manner in which this machine acts 
will be understood, 

498. Suppose Fig. 103 . 

the extremity 1, 
fig. 103, to be 
presented up¬ 
wards, as in the 
figure, the screw 
itself being in¬ 
clined as repre¬ 
sented. Then, 
from its peculiar 
form and posi¬ 
tion, it is evident, 
that commencing at 1, the screw will descend until we ar¬ 
rive at a certain point 2; in proceeding from 2 to 3 it 
will ascend. Thus, 2 is a point so situated that the parts of 
the screw on both sides of it ascend, and therefore if any 
body, as a ball, were placed in the tube at 2, it could not 
move in either direction without ascending. Again, the 
point 3 is so situated, that the tube on each side of it de- 



How may the principle of Archimedes screw be readily illustrated? Ex¬ 
plain the manner in which a ball would ascend fig. 103, by turning the screw 



HYDRAULICS. 


127 

scends ; and as we proceed we find another point 4, which, 
like 2, is so placed, that the tube on both sides of it ascends, 
and, therefore, a body placed at 4, could not move without 
ascending. In like manner, there is a series of other points 
along the tube, from which it either descends or ascends, as 
is obvious by inspection, 

499: Now let us suppose a ball, less in size than the bore 
of the tube, so as to move freely in it, to be dropped in at 1. 
As the tube descends from 1 to 2, the ball of course will de¬ 
scend down to 2, where it will remain at rest. 

Next, suppose the ball to be fastened to the tube at 2, and 
suppose the screw to be turned nearly half round, so that the 
end 1 shall be turned downwards, and the point 2 brought 
nearly to the highest point of the curve 1, 2, 3. 

500. This movement of the spiral, it is evident, would 
change the positions of the ascending and descending parts, 
as represented by fig. 104. 

The ball, which we 
supposed attached to 
the tube, is now nearly 
at the highest point at 
2 , and if detached will 
descend down to 3, 
where it will rest. The 
point at which 2 was 
placed in the first posi¬ 
tion of the screw is 
marked by b , in the sec¬ 
ond position. The ef¬ 
fect of turning the screw, 
therefore, will be to 
transfer the ball from the highest to the lowest point. An¬ 
other half turn of the screw will cause the ball to pass over 
another high point, and descend the declivity down to 5, in 
fig. 101, where it will again rest. 

501. It is unnecessary to explain the steps by which the 
ball would gain another point of elevation, since it is clear 
that by continuing the same process of action, and of reason¬ 
ing, it would be plain that the ball would be gradually trans¬ 
ferred from the lowest to the highest point of the screw. 

Now all that we have said with respect to the ball, would 
be equally true of a drop of water in the tube , and, there¬ 
fore if the extremity of the tube were immersed in water, so 
that the fluid, by its pressure or weight, be continually forced 
into the extremity of the screw, it would, by making it re 


Rig. 104. 



128 


HYDRAULICS. 


Fig. 105. 


volve, be gradually carried along the spiral to any height to 
which it might extend. 

502. It will, however, be seen, from the above explanation, 
that the tube must not be so elevated from the point of im¬ 
mersion, that the spirals will not descend from one point to 
another, in which case it is obvious that the machine will 
not act. If the tube be placed in a perpendicular position, 
the ball, instead of gaining an increased elevation by turning 
the screw, would descend to the ground. A certain inclina¬ 
tion, therefore, depending on the course of the screw, must be 
given this machine, in order to ensure its action. 

503. Rope Machine .—Instead of this 
method, water was sometimes raised 
by the ancients, by means of a rope, or 
bundle of ropes, as shown at fig. 105. 

This mode illustrates in a very strik¬ 
ing manner, the force of friction be¬ 
tween a solid and fluid, for it was by 
this force alone, that the water was 
supported and elevated. 

504. The large wheel «, is supposed 
to stand over the well, and Z>, a smaller 
wheel, is fixed in the water. The rope 
is extended between the two wheels, 
and rises on one side in a perpendicu¬ 
lar direction. On turning the wheel 
by the crank d , the water is brought 

up by the friction of the rope, and falling into a reservoir a 
the bottom of the frame which supports the wheel, is dis¬ 
charged at the spout d. 

It is evident that the motion of the wheel, and consequently 
that of the rope, must be very rapid, in order to raise any 
considerable quantity of water by this method. But when 
the upward velocity of the rope is eight or ten feet per second, 
a large quantity of water may be elevated to a considerable 
height by this machine. 

505. Barker's Mill .—For the different modes of applying 
water as a power for driving mills, and other useful pur¬ 
poses, we must refer the reader to works on practical me¬ 
chanics. There is, however, one method of turning ma¬ 
chinery by water, invented by Dr. Barker, which is strictly 



What is said concerning the inclination of the tube, in order to insure its 
action ? Explain in what manner water is raised by the machine represented 
by fig. 105. 













HYDRAULICS. 


129 


a pmlosophical, and at the same time a most curious inven¬ 
tion, and therefore is properly introduced here. 

506. This machine is called 
Barker's centrifugal mill , and 
such parts of it as are neces¬ 
sary to understand the princi¬ 
ple on which it acts are repre¬ 
sented by fig. 106. 

The upright cylinder a , is a 
tube which has a funnel shap¬ 
ed mouth, for the admission 
of the stream of water from 
the pipe b. This tube is six 
or eight inches in diameter, 
and may be from ten to twenty 
feet long. The arms n and o, 
are also tubes communicating 
freely with the upright one, 
from the opposite sides of 
which they proceed. The 
shaft dj is firmly fastened to 
the inside of the tube, openings 
at the same time being left for 
the water to pass to the arms o and n. The lower part of 
the tube is solid, and turns on a point resting on a block of 
stone or iron, c. The arms are closed at their ends, near 
which are the orifices on the sides opposite to each other, so 
that the water spouting from them, will fly in opposite direc¬ 
tions. The stream from the pipe 5, is regulated by a stop¬ 
cock, so as to keep the tube a constantly full without over¬ 
flowing. 

To set this engine in motion, suppose the upright tube to 
be filled with water, and the arms n and o to be given a 
slight impulse; the pressure of the water from the perpen¬ 
dicular column in the large tube will give the fluid the ve¬ 
locity of discharge at the ends of the arms proportionate to 
its height. The reaction that is produced between the air 
and the water, on the points behind the discharging orifices, 
will continue, and increase the rotatory motion thus begun. 
After a few revolutions, the machine will receive an addi¬ 
tional impulse by the centrifugal force generated in the arms, 
and in consequence of this, a much more violent and rapid 


Fig. 106. 





What is fig. 106 intended to represent ? Describe this mill. 











130 


HYDRAULICS. 


discharge of the water takes place, than would occur by the 
pressure of that in the upright tube alone. The centrifugal 
force, and the force of the discharge thus acting at the same 
time, and each increasing the force of the other, this machine 
revolves with great velocity and proportionate power. The 
friction which it has to overcome, when compared with that 
of other machines, is very slight, being chiefly at the point 
c, where the weight of the upright tube and its contents is 
sustained. 

By fixing a cog wheel to the shaft at d, motion may be 
given to any kind of machinery required. 

507. Where the quantity of water is small, but its height 
considerable, this machine may be employed to great advan¬ 
tage, it being under such circumstances one of the most 
powerful engines ever invented. 

WATER WHEELS. 

508. All water wheels consist of a drum, or hollow cylin¬ 
der, revolving on an axis, while the diameter or exterior 
surface, is covered with float-boards, vanes, or cavities called 
buckets , upon which the water acts, first, to give motion to the 
wheel, and then to machinery. These wheels are of three 
kinds, namely, the over-shot, under-shot, and breast wheels. 

509. Over-shot Wheel. 

-—This wheel of all oth¬ 
ers, gives the greatest 
power with the least 
quantity of water, and 
is, therefore, generally 
used when circumstan¬ 
ces will permit, or where 
there is a considerable 
fall, with a limited quan¬ 
tity of water. The 
over-shot wheel, fig. 

107, requires a fall equal 
to at least its own di¬ 
ameter, and it is cus¬ 
tomary to give it a 
greater length than other wheels, that the cells or buckets 
may contain a large quantity of water, for it is by the weight 
and not the momentum of the fluid that this wheel is turned. 


Fig. 107. 



Of what do all water wheels consist ? How many kinds of water wheels 
are there ? What is the chief advantage of the over-shot wheel ? 







water wheels. 


131 

510. In its construction, the drum, or circumference is 
made water-tight, and to this are fixed narrow troughs or 
buckets, formed of iron, or boards, running the whole length 
of the drum. The water is conducted by a trough nearly 
level, and in width equal to the length of the wheel. It falls 
into the buckets on the top of the wheel, and hence the name 
over-shot. The buckets are so constructed as to retain the 
water until the wheel has made about one-third of a revolu¬ 
tion from the place of admission, when it escapes as from an 
inverted vessel, and the wheel ascends with empty buckets, 
while on the opposite side they are filled with water, and 
thus the revolution is perpetuated. This whole machine 
and its action are so plain and obvious as to require no par¬ 
ticular reference. 

511. From the experiments of Mr. Smeaton, it appears, 
that the fall and quantity of water, and the diameter of the 
wheel being the same, the over-shot will produce about 
double the effect of the under-shot wheel. 

512. Under-skot Wheel. 

—This is so called be¬ 
cause the water passes 
under , instead of over the 
circumference, as in that 
above described. Hence 
it is moved by the mo¬ 
mentum, not the weight 
of the water. Its con¬ 
struction, as shown by 
fig. 108, is different from 
the over-shot, since in¬ 
stead of tight buckets to 
retain the water, it has 
float-hoards standing like rays around the circumference. 
Thus construc ted, this wheel moves equally well whether the 
water acts on one or the other side of the boards, and hence 
is employed for tide wheels, which turn in one direction when 
the tide is going out, and in the other when it is coming in. 

This wheel requires a rapid flow, and a large quantity of 
water, to give it an efficient motion. 


Fig. 108. 



Is this wneei turned by the weight or momentum of the water? Describe 
its construction. What is said of the construction of the buckets ? Circum 
stances being equal, how much greater power has the over-shot than the under¬ 
shot wheel? Where does the water pass in the under-shot wheel? What 
kind ot lorce moves this wheel? What is a tide wheel ? 








132 


PNEUMATICS 


513. Breast Wheel .— 

This wheel, in its con¬ 
struction, or rather in 
the application of the 
moving power, is be¬ 
tween the two wheels 
already described. In 
this the water, instead 
of passing over, or en¬ 
tirely under the wheel, 
is delivered in the direc¬ 
tion of its centre, fig. 

109. This is one of the 
most common wheels, 
and is employed where there is not a sufficient fall for the 
construction of the over-shot kind. 

514. The breast wheel is moved partly by the weight, and 
partly by the momentum of the water. But notwithstanding 
this double force, this wheel is greatly inferior to the over¬ 
shot, in effect, not only because the lever power is dimin¬ 
ished by the smaller diameter, but also on account of the 
great waste of water which always attends the best con¬ 
structed wheels of this kind. 

515. General Remarks. —In order to allow any of the 
above wheels to act with freedom, and to their fullest power, 
it is absolutely necessary that the water which is discharged 
at the bottom of the wheel should have a wide and uninter¬ 
rupted passage to run away, for whenever this is not the 
case it accumulates and forms a resistance to the action of 
the buckets or float-boards, and thus subtracts just so much 
from the velocity and power of the machine. 


Fig. 109. 



PNEUMATICS. 

516. The term Pneumatics is derived from the Greek pneu - 
ma, which signifies breath , or air. It is that science which in¬ 
vestigates the mechanical properties of air , and other elastic 
fluids. 

How does the breast wheel differ from the over-shot and under-shot wheels ? 
Where does the water strike this wheel ? By what power is the breast wheel 
moved? Why is this wheel inferior to the over-shot? What cautions are ne¬ 
cessary m order to permit any of the wheels described to produce their full 
effects? What is pneumatics? 












PNEUMATICS. 


133 


Cinder the article Hydrostatics , (412,) it was slated that 
fluids were of two kinds, namely, elastic and non-elastic , and 
that air and the gases belonged to the first kind while water 
and other liquids belonged to the second. 

517. The atmosphere which surrounds the earth, and in 
which we live, and a portion of which we take into our 
lungs at every breath, is called afr, while the artificial pro¬ 
ducts which possess the same mechanical properties, are 
called gases. 

When, therefore, the word air is used in what follows, 
it will be understood to mean the atmosphere which we 
breathe. 

518. Every hollow, crevice, or pore, in solid bodies, not 
filled with a liquid, or some other substance, appears to be 
filled with air : thus a tube of any length, the bore of which 
is as small as it can be made, if kept open, will be filled with 
air; and hence, when it is said that a vessel is filled with 
air, it is only meant that the vessel is in its ordinary state. 
Indeed, this fluid finds its way into the most minute pores of 
all substances, and cannot be expelled and kept out of any 
vessel, without the assistance of the air-pump, or some other 
mechanical means. 

519. By the elasticity of air is meant its spring, or the force 
with which it re-acts, when compressed in a close vessel. It 
is chiefly in respect to its elasticity and lightness, that the 
mechanical properties of air differ from those of water, and 
other liquids. 

520. Elastic fluids differ from each other in respect to the 
'permanency of the elastic property. Thus, steam is elastic 
only while its heat is continued, and on cooling, returns 
again to the form of water. 

521. Some of the gases, also, on being strongly compressed, 
lose their elasticity, and take the form of liquids. But air 
differs from these, in being permanently elastic; that is, if it 
be compressed with ever so much force, and retained under 
compression for any length of time, it does not therefore lose 
its elasticity, or disposition to regain its former bulk, but al¬ 
ways re-acts with a force in proportion to the power by which 
it is compressed. 


What is air ? What is gas ? What is meant when it is said that a vessel is 
filled with air? Is there any difficulty in expelling the air from vessels? 
What is meant by the elasticity of air? How does air differ from steam and 
some of the gases, in respect to its elasticity. 


12 



134 


PNEUMATICS. 


522. Compression by Experiment. —Thus, if 
the strong tube, or barrel, fig. 110, be smooth, 
and equal on the inside, and there be fitted to 
it me solid piston, or plug a , so as to work up 
and down air tight, by the handle b : the air 
in the barrel may be compressed into a space 
a hundred times less than its usual bulk. In¬ 
deed, if the vessel be of sufficient strength, 
and the force employed sufficiently great, its 
bulk may be lessened a thousand times, or in 
any proportion, according to the force employ¬ 
ed ; and if kept in this state for years, it will 
regain its former bulk the instant the pressure 
is removed. 

Thus, it is a general principle in pneumat¬ 
ics, that air is compressible in proportion to the force em¬ 
ployed. 

523. Expansion of the Air .—On the contrary, when the 
usual pressure of the atmosphere is removed from a portion 
of air, it expands and occupies a space larger than before; 
and it is found by experiment, that this expansion is in a ra¬ 
tio, as the removal of the pressure is more or less complete. 
Air also expands or increases in bulk, when heated. 

If the stop-cock c, fig. 110, be opened, the piston a may 
be pushed down with ease, because the air contained in the 
barrel will be forced out at the aperture. Suppose the piston 
to be pushed down to within an inch of the bottom, and then 
the stop-cock closed, so that no air can enter below it. Now, 
on drawing the piston up to the top of the barrel, the inch 
of air will expand and fill the whole space, and were this 
space a thousand times as large, it would still be filled with 
the expanded air, because the piston removes the pressure of 
the external atmosphere from that within the barrel. 

It follows, therefore, that the space which a given portion 
of air occupies, depends entirely on circumstances. If it is 
under pressure, its bulk will be diminished in exact proportion ■ 
and as the pressure is removed, it w.i\\ expand in proportion, 
so as to occupy a thousand, or even a million times as much 
space as before. 


Fig. 110 



c 


Does air lose its elastic force by being long compressed? In what propor¬ 
tion to the force employed is the bulk of air lessened ? In what proportion 
will a quantity of air increase in bulk as the pressure is removed from it? 
How is this illustrated by fig. 110? On what circumstance, therefore, will the 
bulk of a given portion of air depend ? 











PlNEUMATIUS. 


135 


524 Another property which air possesses is weigh!, or 
gravity. This property, it is obvious, must be slight, when 
compared with the weight of other bodies. But that air has 
a certain degree of gravity in common with other ponderous 
substances, is proved by direct experiment. Thus, if the air 
be pumped out of a close vessel, and then the vessel be ex¬ 
actly weighed, it will be found to weigh more when the air 
is again admitted. 

525. Pressure of the Atmosphere .—It is, however, the 
weight of the atmosphere which presses on every part of the 
earth’s surface, and in which we live and move, as in an 
ocean, that here particularly claims our attention. 

The pressure of the atmosphere may be easi- Fi s- 11L 
ly shown by the tube and piston, fig. 111. , 

Suppose there is an orifice to be opened or 
closed by the valve 6, as the piston a is moved 
up or down in its barrel. The valve being fas¬ 
tened by a hinge on the upper side, on pushing 
the piston down, it will open by the pressure of 
the air against it, and the air will make its es¬ 
cape. But when the piston is at the bottom of b 

the barrel, on attempting to raise it again, to- gM l & 
wards the top, the valve is closed by the force 
of the external air acting upon it. If, therefore, 
the piston be drawn up in this state, it must be -=! 

against the pressure of the atmosphere, the 
whole weight of which, to an extent equal to the diameter of 
the piston, must be lifted, while there will remain a vacuum 
or void space below it in the tube. If the piston be only 
three inches in diameter, it will require the full strength of a 
man to draw it to the top of the barrel, and when raised, if 
suddenly let go, it will be forced back again by the weight 
of the air, and will strike the bottom with great violence. 

526. Supposing the surface of a man to be equal to 14£ 
square feet, and allowing the pressure on each square inch 
to be 15 lbs., such a man would sustain a pressure on his 
whole surface equal to nearly 14 tons. 

527. Now, that it is the weight of the atmosphere which 
presses the piston down, is proved by the fact, that if its diame¬ 
ter be enlarged, a greater force, in exact proportion, will be 



b 


ill 



How is it proved that air has weight? Explain in what manner the pressure 
of the atmosphere is shown by fig. 111. The force pressing on the piston, when 
drawn upward, is sometimes called suction. How is it proved that it is the 
weight of the atmosphere, instead of suction, which makes the piston rise with 
difficulty ? 















106 


aiR PUMP. 


"squired to raise it. And further, if when the piston is 
Irawn to the top of the tube, a stop-cock, as at fig. 110, be 
opened, and the air admitted under it, the piston will not be 
orced down in the least, because then the air will press as 
much on the under, as on the upper side of the piston. 

• r >28. By accurate experiments, an account of which it is 
not necessary here to detail, it is found that the weight of 
the atmosphere on every inch square of the surface of the 
3 arth is equal to fifteen pounds. If, then, a piston working 
air tight in a barrel, be drawn up from its bottom, the force 
amployed, besides the friction, will be just equal to that re¬ 
quired to lift the same piston, under ordinary circumstances, 
with a weight laid on it equal to fifteen pounds for every 
square inch of surface. 

529. The number of square inches in the surface of a pis¬ 
ton of a foot in diameter, is 113. This being multiplied by 
the weight of the air on each inch, which being 15 pounds, 
is equal to 1695 pounds. Thus the air constantly presses 
on every surface, which is equal to the dimensions of a circle 
one foot in diameter, with a weight of 1695 pounds. 

AIR PUMP. 

530. The air pump is an engine by which the air can be 
pumped out of a vessel , or withdrawn from it. The vessel so 
exhausted, is called a receiver, and the space thus left in the 
vessel , after withdrawing the air , is called a vacuum. 

The principles on which the air pump is constructed are rea¬ 
dily understood, and are the same in all instruments of this 
kind, though the form of the instrument itself is often con¬ 
siderably modified. 

531. The general principles 
of its construction will be com¬ 
prehended by an explanation 
of fig. 112. In this figure let 
g be a glass Vessel, or receiver, 
closed at the top, and open at 
the bottom, standing on a per¬ 
fectly smooth surface, which is 
called the plate of the air pump. 

Through the plate is an aperture, 
a, which communicates with the 
inside of the receiver, and the 
barrel of the pump. The piston 
rod, p , works air tight through 


Fig. 112. 



















AIR PUMP. 


137 


the stuffed collar, c, and the piston also moves air tight 
through the barrel. At the extremity of the barrel, there is 
a valve e, which opens outwards, and is closed with a spring. 

532. Now suppose the piston to be drawn up to c , it will 
then leave a free communication between the receiver g 7 
through the orifice «, to the pump barrel in which the piston 
works. Then if the piston be forced down by its handle, it 
will compress the air in the barrel between d and e, and, in 
consequence, the valve e will be opened, and the air so con¬ 
densed will be forced out. On drawing the piston up again, 
the valve will be closed, and the external air not being per¬ 
mitted to enter, a vacuum will be formed in the barrel, from 
e to a little above d. When the piston comes again to c, the 
air contained in the glass vessel, together with that in the 
passage between the vessel and the pump barrel, will rush 
in to fill the vacuum. Thus, there will be less air in the 
whole space, and consequently in the receiver, than at first, 
because all that contained in the barrel is forced out at every 
stroke of the piston. On repeating the same process, that is, 
drawing up and forcing down the piston, the air at each 
time in the receiver will become less and less in quantity, 
and, in consequence, more and more rarefied. For it must 
be understood, that although the air is exhausted at every 
stroke of the pump, that which remains, by its elasticity, ex 
pands, and still occupies the whole space. The quantity 
forced out at each successive stroke is therefore diminished, 
until, at last, it no longer has sufficient force before the pis¬ 
ton to open the valve, when the exhausting power of the in 
strument must cease entirely. 

Now it will be obvious, that as the exhausting power of 
the air pump depends on the expansion of the air within it, 
a perfect vacuum can never be formed by its means, for so 
long as exhaustion takes place, there must be air to be forced 
out, and when this becomes so rare as not to force open the 
valves, then the process must end. 

533. A good air pump has two similar pumping barrels 
to that described, so that the process of exhaustion is per- 


What is the pressure of the atmosphere on every square inch of surface on 
the earth ? What is the number of square inches in a circle of one foot in di¬ 
ameter ? What is the weight of the atmosphere on a surface of a foot in diame¬ 
ter ? What is an air pump ? What is the receiver of an air pump ? What 
is a vacurm? In fig. 112, which is the receiver of the air pumh? When the 
piston is pressed down, what quantity of air is thrown out? When the pis¬ 
ton is drawn up, what is formed in the barrel ? How is this vacuum again fill 
ed with air ? Is the air pump capable of producing a perfect vacuum ? 

12* 



138 


air pump. 


formed in half the time that it could be performed by one 
barrel. 

The barrels, with 
their pistons, and the 
usual mode of work¬ 
ing them, are repre¬ 
sented by fig. 113. 

The piston rods are 
furnished with racks, 
or teeth, and are work¬ 
ed by the toothed 
wheel a , which is 
turned backwards and 
forwards, by the lever 
and handle b. The 
exhaustion pipe c, 
leads to the plate on 
which the receiver 
stands, as shown in 
fig. 113. The valves 
V, n, u, and m, all open upwards. 

534 . To understand how these pistons act to exhaust the 
air from the vessel on the plate, through the pipe c, we will 
suppose, that as the two pistons now stand, the handle b is 
to be turned towards the left. This will raise the piston A , 
while the valve u will be closed by the pressure of the ex¬ 
ternal air acting on it in the open barrel in which it works. 
There would then be a vacuum formed in this barrel, did 
not the valve m open, and let in the air coming from the re¬ 
ceiver, through the pipe c. When the piston, therefore, is 
at the upper end of the barrel, the space between the piston 
and the valve m, will be filled with the air from the receiver. 
Next, suppose the handle to be moved to the right, the pis¬ 
ton A will then descend, and compress the air with which 
the barrel is filled, which, acting against the valve it, forces 
it open, and thus the air escapes. Thus, it is plain, that 
every time the piston rises, a portion of air, however rarefied, 
enters the barrel, and every time that it descends, this portion 
escapes, and mixes with the external atmosphere. 

The action of the other piston is exactly similar to this, 


Why do common air pumps have more than one barrel and piston? How 
are the pistons of an air pump worked ? While the piston A is ascending, 
which valves will be open, and which closed? When the piston A descends, 
what becomes of the air with which its barrel is filled ? 


Fig. 113. 














CONDENSER. 


139 


only that B rises while A falls, and so the contrary. It will 
appear, on an inspection of the figure, that the air cannot 
pass from one barrel to the other, for while A is rising, and 
the valve m is open, the piston B will be descending, so that 
the force of the air in the barrel B, will keep the valve n 
closed. Many interesting and curious experiments, illus¬ 
trating the expansibility and pressure of the atmosphere, are 
shown by this instrument. 

535. If a withered apple be placed under the receiver, 
and the air is exhausted, the apple will swell and become 
plump, in consequence of the expansion of the air which it 
contains within the skin. 

536. Ether, placed in the same situation, soon begins to 
boil without the influence of heat, because its particles, not 
having the pressure of the atmosphere to force them to¬ 
gether, fly off with so much rapidity as to produce ebulition. 

THE CONDENSER. 

537. The operation of the condenser is the reverse of that of 
the air pump , and is a much more simple machine. The air 
pump, as we have just seen, will deprive a vessel of its ordi¬ 
nary quantity of air. The condenser, on the contrary, will 
double or treble the ordinary quantity of air in a close vessel, 
according to the force employed. 

This instrument, fig. 114, consists of a 
pump barrel and piston a , a stop-cock b , and 
the vessel c furnished, with a valve opening 
downwards. The orifice d is to admit the air, 
when the piston is drawn up to the top of the 
barrel. 

538. To describe its action, let the piston 
be above d , the orifice being open, and there¬ 
fore the instrument filled with air, of the same 
density as the external atmosphere. Then, 
on forcing the piston down, the air in the 
pump barrel, below the orifice d, will be com¬ 
pressed, and will rush through the stop-cock, 
b , into the vessel c, where it will be retained, 
because, on again moving the piston upward, 
the elasticity of the air will close the valve 
through which it was forced. On drawing 

Why does not the air pass from one barrel to the other, through the valves m 
and ra. ? Why does an apple placed in the exhausted receiver grow plump? 
Why does ether boil in the same situation? How does the condenser operate ? 
Explain fig. 114, and show in what manner the air is condensed. 


Fig. 114. 


I 













140 


BAROMETER. 


the piston up again, another portion of air will rush in at 
the orifice d, and on forcing it down, this will also be driven 
into t he vessel c; and this process may be continued as 
long as sufficient force is applied to move the piston, or 
mere is sufficient strength in the vessel to retain the air. 
When the condensation is finished, the stop-cock b may be 
turned, to render the confinement of the air more secure. 

539. Air Gun .—The magazines of air guns are filled in 
the manner above described. The air gun is shaped like 
other guns, but instead of the force of powder, that of air is 
employed to project the bullet. For this purpose, a strong 
hollow ball of copper, with a valve on the inside, is screwed 
to a condenser, and the air is condensed in it, thirty or forty 
times. This ball or magazine is then taken from the con¬ 
denser, and screwed to the gun, under the lock. By means 
of the lock, a communication is opened between the maga¬ 
zine and the inside of the gun-barrel, on which the spring of 
the confined air against the leaden bullet is such as to throw 
it with nearly the same force as gunpowder. 

B A ROMETER. 

540. Suppose a, fig. 115, to be a long 
tube, with the piston b so nicely fitted to its 
inside, as to work air tight. If the lower 
end of the tube be dipped into water, 
and the piston drawn up by pulling at the 
handle c, the water will follow the piston so 
closely, as to be in contact with its surface, 
and apparently to be drawn up by the pis¬ 
ton, as though the whole was one solid body. 

If the tube be thirty-five feet long, the water 
will continue to follow the piston, until it 
comes to the height of about thirty-three 
feet, where it will stop, and if the piston be 
drawn up still farther, the water will not 
follow it, but will remain stationary, the space 
from this height, between the piston and the 
water, being left a void space, or vacuum. 

541. The rising of the water in the above 
case, which only involves the principle of the 
common pump, is thought by some to be 
caused by suction, the piston sucking up the 
water as it is drawn upward. But accord¬ 
ing to the common notion attached to this 


Fig. 115. 



















BAROMETER. 


141 


term, there is no reason why the water should not continue 
to rise above the thirty-three feet, or why the power of suc¬ 
tion should cease at that point, rather than at any other. 
Without entering into any discussion on the absurd notions 
concerning the power of suction, it is sufficient here to state, 
that it has long since been proved, that the elevation of the 
water, in the case above described, depends entirely on the 
weight and pressure of the atmosphere, on that portion of the 
fluid which is on the outside of the tube. Hence, when the 
piston is drawn up, under circumstances where the air can¬ 
not act on the water around the tube, or pump barrel, no 
elevation of the fluid will follow. This will be obvious, by 
the following experiment. 

542. Proof that the pump acts hy external Fig. 116. 

pressure .—Suppose fig. 116 to be the sections 
or halves, of two tubes, one within the other, 
the outer one being made entirely close, so as 
to admit no air, and the space between the 
two being also made air tight at the top. 

Suppose, also, that the inner tube being left 
open at the lower end, does not reach the 
bottom of the outer tube, and thus that an 
open space be left between the two tubes eve¬ 
ry where, except at their upper ends, where 
they are fastened together; and suppose that 
there is a valve in the piston, opening up¬ 
wards, so as to let the air which it contains 
escape, but which will close on drawing the 
piston upwards. Now, let the piston be at a, 
and in this state pour water through the 
stop-cock, c, until the inner tube is filled up 
to the piston, and the space between the two 
tubes filled up to the same point, and then let 
the stop-cock be closed. If now the piston 
be drawn up to the top of the tube, the water 
will not follow it, as in the case first describ- IlHllI b 
ed ; it will only rise a few inches, in conse- 


Explain the principle of the air gun. Suppose the tube, fig. 115, to stand 
with its lower end in the water, and the piston a to be drawn upward thirty-five 
feet, how far will the water follow the piston ? What will remain in the tube 
between the piston and the water, after the piston rises higher than thirty-three 
feet ? What is commonly supposed to make the water rise in such cases ? 
Is there any reason why the suction should cease at thirty-three feet ? What 
is the true cause of the elevation of the water, when the piston, fig. 115, is 
drawn up ? How is it shown by fig. 116, that it is the pressure of the atmos 
phere which causes the water to rise in the pump barrel ? 
















BAROMETER. 


! 4*2 

quence of the elasticity of the air above the water, between 
the tubes, and in the space above the water, there will be 
formed a vacuum between the water and the piston, in the 
inner tube. 

543. The reason why the result of this experiment differs 
from that before described, is, that the outer tube prevents the 
pressure of the atmosphere from forcing the water up the 
inner tube as the piston rises. This may be instantly proved, 
by opening the stop-cock c, and permitting the air to press 
upon the water, when it will be found, that as the air rushes 
in, the water will rise and fill the vacuum, up to the piston. 

For the same reason, if a common pump be placed in a 
cistern of water, and the water is frozen over on its surface, 
so that no air can press upon the fluid, the piston of the 
pump might be worked in vain, for the water would not, as 
usual, obey its motion. 

544. It follows, as a certain conclusion from such experi¬ 
ments, that when the lower end of a tube is placed in water, 
agd the air from within removed by drawing up the piston, 
that it is the pressure of the atmosphere on the water around 
the tube, which forces the fluid up to fill the space thus left 
by the air. It is also proved, that the weight, or pressure of 
the atmosphere, is equal to the weight of a perpendicular 
column of water 33 feet high, for it is found (fig. 115) that 
the pressure of the atmosphere will not raise the water more 
than 33 feet, though a perfect vacuum be formed to any 
height above this point. Experiments on other fluids, prove 
that this is the weight of the atmosphere, for if the end of 
the tube be dipped in any fluid, and the air be removed from 
the tube, above the fluid, it will rise to a greater or less 
height than water, in proportion as its specific gravity is less, 
or greater than that of water. 

545. Mercury, or quicksilver, has a specific gravity of 
about 13 J times greater than that of water, and mercury is 
found to rise about 29 inches in a tube under the same cir¬ 
cumstances that water rises 33 feet. Now, 33 feet is 396 
inches, which being divided by 29, gives nearly 131, so that 
mercury being 131 times heavier than water, the water will rise 
under the same pressure 13£ times higher than the mercury. 


Suppose the ice prevents the atmosphere from pressing on the water in a 
vessel, can the water be pumped out ? What conclusion follows from the ex¬ 
periments above described ? How is it proved, that the pressure of the atmos 
phere is equal to the weight of a column of water 33 feet high ? How do ex¬ 
periments on other fluids show that the pressure of the atmosphere is equal to 
the weight of a column of water, 33 feet high ? How high does mercury rise 
in an exhausted tube ? 



BAROME' 


143 


546. Construction of the Barometer .— 

The barometer is constructed on the prin¬ 
ciple of atmospheric pressure, which we 
have thus endeavored to explain and il¬ 
lustrate to common comprehension. 

This term is compounded of two Greek 
words, baros , weight, and metron, meas¬ 
ure, the instrument being designed to 
measure the weight of the atmosphere. 

Its construction is simple, and easily 
understood, being merely a tube of glass, 
nearly filled with mercury, with its low¬ 
er end placed in a dish of the same fluid, 
and the upper end furnished with a scale, 
to measure the height of the mercury. 

547. Let a, fig. 117, be such a tube, 

34 or 35 inches long, closed at one end, and open at the oth¬ 
er. To fill the tube, set it upright, and pour the mercury in 
at the open end, and when it is entirely full, place the fore 
finger forcibly on this end, and then plunge the tube and fin¬ 
ger under the surface of the mercury, before prepared in the 
cup b. Then withdraw the finger, taking care that in doing 
this, the end of the tube is not raised above the mercury in 
the cup. When the finger is removed, the mercury will de¬ 
scend four or five inches, and after several vibrations, up and 
down, will rest at an elevation of 29 or 30 inches above the 
surface of that in the cup, as at c. Having fixed a scale to 
the upper part of the tube, to indicate the rise and fall of the 
mercury, the barometer would be finished, if intended to re¬ 
main stationary. It is usual, however, to have the tube in¬ 
closed in a mahogany or brass case, to prevent its breaking, 
and to have the cup closed on the top, and fastened to the 
tube, so that it can be transported without danger of spilling 
the mercury. 

548. The cup of the portable barometer also differs from 
that described, for were the mercury inclosed on all sides, in 
a cup of wood, or brass, the air would be prevented from act¬ 
ing upon it, and therefore the instrument would be useless. 
To remedy this defect, and still have the mercury perfectly 



What is the principle on which the barometer is constructed ? What does 
the barometer measure ? Describe the construction of the barometer, as repre¬ 
sented by fig. 117. How is the cup of the portable barometer made so as to 
retain the mercury, and still allow the air to press upon it ? What is the use 
of the metallic plate and screw, under the bottom of the cup? 










144 


BA.ROMETER. 


inclosed, the bottom of the cup is made of leather, which, be¬ 
ing elastic, the pressure of the atmosphere acts upon the mer¬ 
cury in the same manner as though it was not inclosed at 
all. Below the leather bottom, there is a round plate of met¬ 
al, an inch in diameter, which is fixed on the top of a sciew, 
so that when the instrument is to be transported, by eleva¬ 
ting this piece of metal, the mercury is thrown up to the top 
of the tube, and thus kept from playing backwards and for¬ 
wards, when the barometer is in motion. 

549. A person not acquainted with the principle of the in¬ 
strument, on seeing the tube turned bottom upwards, will be 
perplexed to understand why the mercury does not follow the 
common law of gravity, and descend into the cup; were the 
tube of glass 33 feet high, and filled with water, the lower 
end being dipped into a tumbler of the same fluid, the won¬ 
der would be still greater. But as philosophical facts, one is 
no more wonderful than the other, and both are readily ex¬ 
plained by the principles above illustrated. 

550. Water Barometer .—It has already been shown, 
(542,) that it is the pressure of the atmosphere on the fluid 
around the tube, by which the fluid within it is forced up¬ 
ward, when the pump is exhausted of its air. The pressure 
of the air, we have also seen, is equal to a column of water 
33 feet high, or of a column of mercury 29 inches high. 
Suppose, then, a tube 33 feet high is filled with water, the 
air would then be entirely excluded, and were one of its ends 
closed, and the other end dipped in water, the effect would 
be the same as though both ends were closed, for the w^ater 
would not escape, unless the air were permitted to rush in 
and fill up its place. The upper end being closed, the aii 
could gain no access in that direction, and the open end be 
ing under water, is equally secure. The quantity of water 
in which the end of the tube is placed, is not essential, since 
the pressure of a column of water, an inch in diameter, provi¬ 
ded it be 33 feet high, is just equal to a column of air of an 
inch in diameter, of the whole height of the atmosphere. 
Hence the water on the outside of the tube serves merely tc 
guard against the entrance of the external air. 

551. The same happens to the barometer tube, when filled 
with mercury. The mercury, in the first place, fills the tube 


Explain the reason why the mercury does not fall out of the barometer tube, 
when its open end is downwards. What fills the space above 29 inches, in 
the barometer tube T In the common barometer, how is the rise and fall of th® 
mercury indicated? 



BAROMETER. 


145 


perfectly, and therefore entirely excludes the air, so that 
when it is inverted in the cup, all the space above 29 inches 
is left a vacuum. The same effect precisely would be pro¬ 
duced, were the tube exhausted of its air, and the open end 
placed in the cup; the mercury would run up the tube 29 
inches, and then stop, all above that point being left a va¬ 
cuum. 

The mercury, therefore, is prevented from falling out of the 
tube, by the pressure of the atmosphere on that which re¬ 
mains in the cup ; for if this be removed, the air will enter, 
while the mercury will instantly begin to descend. 

552. Wheel Barometer .—In the barometer described, the 
rise and fall of the mercury is indicated by a scale of inches, 
and tenths of inches, fixed behind the tube ; but it has been 
found that very slight variations in the density of the atmos¬ 
phere are not readily perceived by this method. It being, 
however, desi .>ble that these minute changes should be ren¬ 
dered more ob 'ious, a contrivance for increasing the scale, 
called the wheel barometer, was invented. 

553. The whole length of the tube of the 
wheel barometer, fig. 118, from c to a, is 34 
or 35 inches, and it is filled with mercury, 
as usual. The mercury rises in the short 
leg to the point o , where there as a small 
piece of glass floating on its surface, to 
which there is attached a silk string, pass¬ 
ing over the pulley p. To the axis of the 
pulley is fixed an index, or hand, and be¬ 
hind this is a graduated circle, as seen in 
the figure. It is obvious, that a very slight 
variation in the height of the mercury at o, 
will be indicated by a considerable motion 
of the index, and thus changes in the weight 
of the atmosphere, hardly perceptible by the 
common barometer, will become quite ap¬ 
parent by this. 

554. Heights measured by the Barometer .— 

The mercury in the barometer tube being 
sustained by the pressure of the atmosphere, 
and its medium altitude at the surface of the earth being 
about 29 inches, it might be expected that if the instrument 


Fig. 118 . 
a 



Why was the wheel barometer invented? Explain fig. 118 , and describe 
the construction of the wheel barometer. What is stated to be the medium 
range of the barometer at the surface of the earth ? 

13 










146 


BAROMETER. 


was carried to a height from the earth’s surface, the mercury 
would suffer a proportionate fall, because the pressure must 
be less at a distance from the earth, than at its surface, and 
experiment proves this to be the case. When, therefore, this 
instrument is elevated to any considerable height, the descent 
of the mercury becomes perceptible. Even when it is car¬ 
ried to the top of a hill, or high tower, there is a sensible de¬ 
pression of the fluid, so that the barometer is employed to 
measure the height of mountains, and the elevation to which 
balloons ascend from the surface of the earth. On the top 
of Mont Blanc, which is about 16,000 feet above the level of 
the sea, the medium elevation of the mercury in the tube is 
only 14 inches, while on the surface of the earth, as above 
stated, it is 29 inches. 

555. The medium range of the barometer in several coun¬ 
tries, has generally been stated to be about 29 inches. It 
appears, however, from observations made a* Cambridge, in 
Massachusetts, for the term of 22 years, that its range there 
was nearly 30 inches. 

556. Use of the Barometer .—While the barometer stands 
in the same place, near the level of the sea, the mercury sel¬ 
dom or never falls below 28 inches, or rises above 31 inches, 
its whole range, while stationary, being only about 3 inches. # 

These changes in the weight of the atmosphere, indicate 
corresponding changes in the weather, for it is found, by 
watching these variations in the height of the mercury, that 
when it falls, cloudy or falling weather ensues, and that 
when it rises, fine clear weather may be expected. During 
the time when the weather is damp and lowering, and the 
smoke of chimneys descends towards the ground, the mer¬ 
cury remains depressed, indicating that the weight of the 
atmosphere, during such weather, is less than it is when the 
sky is clear. This contradicts the common opinion, that the 
air is the heaviest when it contains the greatest quantity of 
fog and smoke, and that it is the uncommon weight of the 
atmosphere which presses these vapors towards the ground. 

A little consideration will show, that in this case the popular 
belief is erroneous, for not only the barometer, but all th£ 


Suppose the instrument is elevated from the earth, what is the effect on the 
mercury ? How does the barometer indicate the height of mountains ? What 
is the medium range of the mercury on Mont Blanc ? What is stated to be 
the medium range of the barometer at Cambridge ? How many inches does a 
fixed barometer vary in height ? When the mercury falls, what kind of weather 
is indicated ? When the mercury rises what kind of weather may be expected ? 
When fog and smoke descend towards the ground, is it a sign of a light or 
heavy atmosphere ? 




BAROMETER. 


147 


experiments we have detailed on the subject of specific grav¬ 
ity, tend to show that the lighter any fluid is, the deeper any 
substance of a given weight will sink in it. Common ob¬ 
servation ought, therefore, to correct the error, for every body 
knows that a heavy body will sink in water while a light 
one will swim, and by the same kind of reasoning ought to 
consider, that the particles of vapor would descend through 
a light atmosphere, while they would be pressed up into the 
higher regions by a heavier air. 

557. Use at Sea .—The principal use of the barometer, is 
on board of ships, where it is employed to indicate the ap¬ 
proach of storms, and thus to give an opportunity of prepar¬ 
ing accordingly ; and it is found that the mercury suffers a 
most remarkable depression before the approach of violent 
winds, or hurricanes. The watchful captain, particularly in 
southern latitudes, is always attentive to this monitor, and 
when he observes the mercury to sink suddenly, takes his 
measures without delay to meet the tempest. During a vio¬ 
lent storm, we have seen the wheel barometer sink a hun¬ 
dred degrees in a few hours. But we cannot illustrate the 
use of this instrument at sea better than to give the following 
extract from Dr. Arnot, who was himself present at the time. 
u It was,” he says, “ in a southern latitude. The sun had 
just set with a placid appearance, closing a beautiful after¬ 
noon, and the usual mirth of the evening watch proceeded, 
when the captain’s orders came to prepare with all haste for 
a storm. The barometer had begun to fall with appalling 
rapidity. As yet, the oldest sailors had not perceived even 
a threatening in the sky, and were surprised at the extent 
and hurry of the preparations; but the required measures 
were not completed, when a more awful hurricane burst upon 
them, than the most experienced had ever braved. Nothing 
could withstand it; the sails, already furled, and closely 
bound to the yards, were riven into tatters; even the bare 
yards and masts were in a great measure disabled; and at 
one time the whole rigging had nearly fallen by the board. 
Such, for a few hours, was the mingled roar of the hurricane 
above, of the waves around, and the incessant peals of thun¬ 
der, that no human voice could be heard, and amidst the 
general consternation, even the trumpet sounded in vain. 


By what analogy is it shown that the air is lightest when filled with vapor? 
Of what use is the barometer on hoard of ships ? When does the mercury suf¬ 
fer the most remarkable depression ? What remarkable instance is stated, 
where a ship seemed to be saved by the use of the barometer ? 



143 


PUMP. 


On that awful night, hut for a little tube of mercury which 
had given the warning, neither the strength of the noble ship, 
nor the skill and energies of her commander, could have 
saved one man to tell the tale.” 


PUMPS. 

558. There is a philosophical experiment, of which no one 
in this country is ignorant. If one end of a straw be intro¬ 
duced into a barrel of cider, and the other end sucked with 
the mouth, the cider will rise up through the straw and may 
be swallowed. 

The principles which this experiment involves are exactly 
the same as those concerned in raising water by the pump. 
The barrel of cider answers to the well, the straw to the 
pump log, and the mouth acts as the piston, by which the 
air is removed. 

559. The efficacy of the common pump, in raising water, 
depends upon the principle of atmospheric pressure, which 
has been fully illustrated under the articles air pump and ba¬ 
rometer. 

560. These machines are of three kinds, namely, the suck¬ 
ing , or common pump, the lifting pump, and the forcing pump. 

561. Common Pump. —The common, 
or household pump is most in use, 
and for ordinary purposes, the most 
convenient. It consists of a long tube, 
or barrel, called the pump log , which 
reaches from a few feet above the 
ground to near the bottom of the well. 

At a, fig. 119, is a valve, opening up¬ 
wards, called the pump box. When 
the pump is not in action this is always 
shut. The piston 6, has an aperture 
through it, which is closed by a valve 
also opening upwards. 

By the pupil who has learned what 
has been explained under the articles 
air pump, and barometer, the action of 
this machine will be readily understood. 

562. Suppose the piston b to be 

What experiment is stated, as illustrating the principle of the common pump? 
On what does the action of the common pump depend ? How many kinds of 
pumps are mentioned ? Which kind is the common ? Describe the common 
pump. Explain how the common pump acts. When the lever is depressed, 
what takes place in the pump barrel ? When it is elevated, what takes place ? 













puMr. 


149 

down to a , then on depressing the lever c , a vacuum would 
be formed between a and b , did not the water in the well 
rise, in consequence of the pressure of the atmosphere on that 
around the pump log in the well, and take the place of the 
air thus removed. Then, on raising the end of the lever, 
the valve a closes, because the water is forced upon it, in 
consequence of the descent of the piston, and at the same 
time the valve in the piston b opens, and the water, which 
cannot descend, now passes above the valve b. Next, on 
raising the piston, by again depressing the lever, this portion 
of water is lifted up to b, or a little above it, while another 
portion rushes through the valve a to fill its place. After a 
few strokes of the lever, the space from the piston b to the 
spout, is filled with the water, where, on continuing to work 
the lever, it is discharged in a constant stream. 

Although, in common language, this is called the suction 
pump, still it will be observed that the water is elevated by 
suction , or, in more philosphical terms, by atmospheric 
pressure, only above the valve a, after which it is raised by 
lifting up to the spout. The water, therefore, is pressed 
into the pump barrel by the atmosphere, and thrown out by 
lifting. 

563. Lifting Pump .—The lifting pump , properly so called, 
has the piston in the lower end of the barrel, and raises the 
water through the whole distance, by forcing it upward, 
without the agency of the atmosphere. 

564. In the suction pump , the pressure of the atmosphere 
will raise the water 33 or 34 feet, and no more, after which 
it may be lifted to any height required. 

565. Forcing Pump .—The forcing pump differs from both 
these, in having its piston solid, or without a valve, and also 
in having a side pipe, through which the water is forced, 
instead of rising in a perpendicular direction, as in the others. 

566. The forcing pump is represented by fig. 120, where a 
is a solid piston, working air tight in its barrel. The tube c 
leads from the barrel to the air vessel d. Through the pipe 
p, the water is thrown into the open air. g is a gauge, by 
which the pressure of the water in the air vessel is ascer¬ 
tained. Through the pipe t, the water ascends into the 
barrel, its upper end being furnished with a valve opening 
upwards. 


How far is the water raised by atmospheric pressure, and how <•- ^ dicing r 
How does the lifting pump differ from the common pump? Hjw does the 
forcing pump differ from the common pump? 

13* 



150 


PUMP. 


567. To explain the ac¬ 
tion of this pump, suppose 
the piston to be down to the 
bottom of the barrel, and 
then to be raised upward by 
the lever l; the tendency to 
form a vacuum in the bar¬ 
rel, will bring the water up 
through the pipe i, by the 
pressure of the atmosphere. 

Then, on depressing the 
piston the valve at the bot¬ 
tom of the barrel will be 
closed, and the water, not 
finding admittance through 
the pipe whence it came, 
will be forced through the 
pipe c, and opening the valve 
at its upper end, will enter 
into the air vessel d, and be 
discharged through the pipe 
p , into the open air. 

The water is therefore elevated to the piston barrel by 
the pressure of the atmosphere, and afterwards thrown out 
by the force of the piston. It is obvious, that by this ar¬ 
rangement, the height to which this fluid may be thrown, 
will depend on the power applied to the lever, and the strength 
with which the pump is made. 

The air vessel d contains air in its upper part only, the 
lower part, as we have already seen, being filled with water. 
The pipe p, called the discharging pipe, passes down into 
the water, so that the air cannot escape. The air is there¬ 
fore compressed, as the water is forced into the lower part 
of the vessel, and re-acting upon the fluid by its elasticity, 
throws it out of the pipe in a continued stream.. The con¬ 
stant stream which is emitted from the direction pipe of the 
fire engine, is entirely owing to the compression and elasticity 
of the air in its air vessel. In pumps, without such a vessel, ' 
as the water is forced upwards, only while the piston is act 



Explain fig. 120, and show in what manner the water is brought up through 
the pipe i, and afterwards thrown out at the pipe p. Why does not the air 
escape from the air vessel in this pump ? What effect does the air vessel have 
on the stream discharged ? Why does the air vessel render the labor of raising 
the water more easy f 


















FIRE ENGINE. 


151 


mg upon it, there must be an interruption of the stream while 
the piston is ascending, as in the common pump. The air 
vessel is a remedy for this defect, and is found also to render 
the labor of drawing the water more easy, because the force 
with which the air in the vessel acts on the water, is always 
in addition to that given by the force of the piston, 

FIRE ENGINE. 

568. Theatre engine is a modification of the forcing pump. 
It consists of two such pumps, the pistons of which are 
moved by a lever with equal arms, the common fulcrum 
being at c, fig. 121. While the piston a is descending, the 
other piston, b , is ascending. 

The water is forced by the 
pressure of the atmosphere, 
through the common pipe 
p, and then dividing, as¬ 
cends into the working bar¬ 
rels of each piston, where 
the valves, on both sides, 
prevent its return. By the 
alternate depression of the 
pistons, it is then forced into 
the air box d , and then by 
the direction pipe e , is 
thrown where it is wanted. 

This machine acts precise¬ 
ly like the forcing pump, 
only that its power is dou¬ 
bled, by having two pistons 
instead of one. 

569. Fountain of Hiero .—There is a beautiful fountain, 
called the fountain of Hiero , which acts by the elasticity of 
the air, and on the same principle as that already described. 
Its construction will be understood by fig. 122, but its form 
may be varied according to the dictates of fancy or taste. 
The boxes a and b, together with the two tubes, are made 
air tight, and strong, in proportion to the height it is desired 
the fountain should play. 

570. To prepare the fountain for action, fill the box a, through 
the spouting tube, nearly full of water. The tube c, reaching 



Explain fig. 121, and describe the action of the fire engine. What causes 
the continued stream from the direction pipe of this engine ? How is the foun- 
in of Hiero constructed 7 
















152 


STFAM ENGINE. 


nearly to the top of the box, will 
prevent the water from passing 
downwards, while the spouting 
pipe will prevent the air from 
escaping upwards, after the ves¬ 
sel is about half filled with water. 

Next, shut the stop-cock of the 
spouting pipe, and pour water 
into the open vessel d. This 
will descend into the vessel b, 
through the tube e, which nearly 
reaches its bottom, so that after 
a few inches of water are poured 
in, no air can escape, except by 
the tube c, up into the vessel a. 

The air will then be compressed 
by the weight of the column of 
water in the tube e, and therefore 
the force of the water from the jet 
pipe will be in proportion to the 
height of this tube. If this tube is 20 or 30 feet high, on 
turning the stop-cock, a jet of water will spout from the pipe 
that will amuse and astonish those who have never before 
seen such an experiment. 

STEAM ENGINE. 

571. Like most other great and useful inventions, the 
steam engine, from a very simple contrivance, for the purpose 
of raising water, has been improved at various times, and by 
a considerable number of persons, until it has been brought 
to its present state of power and perfection. 

572. By most writers, the origin of this invention is at¬ 
tributed to the Marquis of Worcester, an Englishman, in 
about 1663. But as he has left no drawing, nor such a par¬ 
ticular description of his machine, as to enable us to define 
its mode of action, it is impossible at the present time, to say 
how much credit ought to be attributed to this invention. 

573. It is certain that the first engines had neither cylin¬ 
ders, piston, nor gearing, by which machinery was made to 
revolve, these most important parts having been added by 
succeeding inventors and improvers. 


Fig. 122. 



On what will the height of the jet from Hiero’s fountain depend? What 
was the origin of the steam engine? To whom is this invention generallf 
attributed ? 



























STEAM ENGINE. 


153 


574 Captain Savarfs Engine.-^*-' The first steam engine 
of which we have any definite description, was that invented 
by Capt. Thomas Savary, an Englishman, in 1698. By this 
engine the water was raised to a certain height, by means 
of a vacuum formed by the condensation of steam, and then 
was forced upward by the direct force of steam from the 
boiler. 

575. It appears that the idea of forming a vacuum by the 
condensation of steam, was suggested to Capt. Savary by 
the following circumstances: 

Having drank a flask of Florence wine at an inn, he 
threw the empty flask on the fire, and a moment after called 
for a basin of water to wash his hands. A small quantity 
of the wine which remained in the flask, began to boil and 
steam issued from its mouth. Observing this, it occurred to 
him to try what effect would be produced by inverting the 
flask, and plunging its mouth into the cold water of the 
basin. Putting on a thick glove to defend his hand from 
the heat, he seized the flask, and the moment he plunged its 
mouth into the water, the liquid rushed up, and nearly filled 
the vessel. 

576. Savary states, that this circumstance suggested im¬ 
mediately to him the possibility of giving effect to the atmos¬ 
pheric pressure, by creating a vacuum by the condensation 
of steam. His plan was to lift the water from the mines to 
a certain height, in this manner, and to force it to the eleva¬ 
tion required by the direct power of the steam. 

577. Fig. 123 will show the principle, though not the pre¬ 
cise form, of Savary’s steam engine. It consists of a boiler , 
a, for the generation of steam, which is furnished with a safety 
valve , b , which opens and lets off the steam, when the pres¬ 
sure would otherwise endanger the bursting of the boiler. 
From the boiler there proceeds the steam pipe, furnished with 
the stop-cock c, to the steam vessel , d. From the bottom of the 
steam vessel, there descends the pipe e, called the suction 
pipe , which dips into the well, or reservoir, from which the 
water is to be raised. This pipe is furnished with a valve, 
opening upwards, at its upper end. From the upper end of 
the steam vessel rises another pipe, f called the force pipe , 
which also has a valve opening upwards. To this pipe is at- 


Who was the inventor of the first engine of which we have any definite 
description ? What was the origin of Capt. Savary’s idea of raising water by 
a vacuum ? What are the parts of which Savary’s engine consisted ? De¬ 
scribe the process by which water is raised from the well to the steam vessel 
with this engine 



154 


STEAM ENGINE. 


tached a small cistern, g , 
furnished with a short pipe, 
called the condensing pipe , 
and from which cold water 
can be drawn, so as to fall 
upon the steam vessel d. 

578. To trace the action 
of this simple apparatus, 
suppose the steam vessels 
and tubes to be filled with 
atmospheric air, which of 
course would be the case, 
while the whole remains 
cold. But on making a 
fire under the boiler, steam 
is generated, which, on 
turning the stop-cock c, is 
let into the steam vessel d , 
where for a time it is con¬ 
densed, and falls down in 
drops on the sides of the 
vessel. The continued sup¬ 
ply of steam will, however, 
soon heat the vessel, so that no more vapor will be con¬ 
densed, and its elastic force will open the upper valve, and 
it will pass off through the pipe f while, at the same time, 
and by the same force, the lower valve will be closed. 

579. When the steam has driven all the atmospheric air 
from the vessel d, and the upper pipe, and there remains 
nothing in them but the pure vapor of water, suppose the 
stop-cock c to be turned, so as to stop the further supply of 
steam, and that at the same time cold water be allowed to 
run from the condensing cistern g, on the steam vessel d. 
The steam will thus be condensed into water, leaving the 
interior of the vessel a vacuum. The pressure of the atmos¬ 
phere will close the upper valve, while the same pressure 
acting on the water surrounding the tube in the well, will 
force the fluid up to take the place of the vacuum in the 
steam vessel d. 

580. The height to which water may thus be elevated, 
we have already seen, is about 33 feet, provided the vacuum 


Fig. 123. 



How high did Savary’s engine elevate water by atmospheric pressure ? De¬ 
scribe the manner in which the water was elevated above the steam vessel. 












STEAM ENGINE. 


155 


bo jjcrfect, but Savary was never able to elevate it more than 
20 feet by this method. 

Wc now suppose that the steam vessel is filled with watei, 
by the creation of a vacuum, and the pressure of the atmos¬ 
phere alone, the direct force of the steam having no agency 
in the process. But in order to continue the elevation above 
the level of the steam vessel, the elastic pressure of the steam 
must be employed. 

581. Let us now suppose, therefore, that the vessel d is 
nearly full of water, and that the stop-cock c is turned, so as 
to admit the steam from the boiler through the tube to the 
upper part of the steam vessel, and consequently above the 
water. At first, the steam will be condensed by the cold 
surface of the water, but as hot water is lighter than cold, 
there will soon become a film of heated liquid, by the con¬ 
densation of the steam on the surface of the cold, so that, in 
a few minutes, no more steam will be condensed. Then the 
direct force of the steam pressing upon the water, will drive 
it through the force pipe f and opening the valve, will ele¬ 
vate it to the height required. 

582. When all the water has been driven out, the con¬ 
tinued influx of the steam will heat the vessel until no farther 
condensation will take place, and the vessel will be filled 
with the pure vapor of water, as before, when the steam be¬ 
ing shut off, and the cold water let on, a vacuum will be 
produced, and another portion of water be elevated to take 
its place, as already described, and so on continually. 

This machine, though a mere apology for the complex and 
effective steam engines of the present day, is nevertheless 
highly creditable to the mechanical genius of the inventor, 
considerng the low state of science and mechanical know¬ 
ledge at ihat time. 

583. These engines were chiefly employed in the drainage 
of the coal mines, and were sufficiently powerful to elevate 
the water to the height of about 90 feet including both the 
atmospheric pressure, and the direct force of the steam. But 
the process was exceeding slow; the quantity of steam 
wasted, consequently, was very great, and the quantity of 
fuel consumed immense. Besides thest disadvantages, the 
bursting power of the steam, when applied v 'flh a force suf¬ 
ficient to elevate a column of water 60 feet high, was such 
as to require vessels of great strength, and, consequently, 


What is said of Savary’s invention ? What were the chief objections to 
Savary’s engines ? 





J56 


STEAM ENGINE. 


engines of small capacity only could be employed. In ad¬ 
dition to these defects, where the mine was several hundred 
feet deep, three or four engines must be employed, since each 
could elevate the water only about 90 feet. It is hardly ne¬ 
cessary, therefore, to say, that Savary’s engine did not an¬ 
swer the principal object of its design, that of draining the 
English mines. 

584. Newcomen's Engine .—The steam engine which suc¬ 
ceeded that of Savary, was invented by Thomas Newcomen, 
a blacksmith, of Dartmouth, in England. Newcomen’s pa¬ 
tent was dated 1707, and in it Capt. Savary was united, in 
consequence of his discovery of the method of forming a va¬ 
cuum by the condensation of steam as already described. 

585. The great object of Newcomen’s invention, like that 
of Savary, was to drain the English mines. To do this, he 
proposed to connect one arch head of a working beam to a 
pump rod, while the other arch head should be connected 
with a piston and rod moving in a cylinder, which piston 
should be made to descend by the pressure of the atmosphere, 
an consequence creating a vacuum under it by the con¬ 
densation of st un. When the piston had been made to de¬ 
scend in this manner, by which the pump at the other end 
of the beam was to be worked, the piston was again to be 
drawn up by the weight of the pump rod, so that this engine 
was moved alternately by means of a vacuum at one end of 
the beam, and a weight at the other. 

586. This was the first proposition which had been made 
to work a piston by means of steam, or rather by means of 
a vacuum, created by the condensation of steam, and may 
be considered as the origin of the present mode of working 
all steam engines. 

587. It is proper to distinguish this as the atmospheric en¬ 
gine , since its movement depended on the pressure of the 
atmosphere alone. 

The adjoining cut, fig. 124, and the following description, 
will show the plan and movement of Newcomen’s engine. 

The boiler a, furnished with a safety valve on the top, has - 
a steam pipe, 6, proceeding to the cylinder d. The piston c 
is of solid metal, and works air tight in the cylinder. The 
piston is attached by its rod to the arch head of the working 


Whose steam engine succeeded that of Savary ? At what, time was New 
comen’s engine invented ? In what manner was Newcomen’s engine worked'» 
What is said of the originality of this invention? Why is Newcomen’s dis 
tmguished by the name of the atmospheric engine ? 



STEAM ENGINE. 


157 


Fig. 124. 



beam f To the other arch head is attached the pump rod 
g , which is connected with its piston to the pump k. This 
pump descends to the water, to be drawn up by the action of 
the engire. The small forcing pump h is supplied with wa¬ 
ter by the pump A, and is designed to raise a portion of the 
fluid through the condensing pipe i, to the cylinder by which 
the steam is condensed. This pump, as well as the other, is 
worked by the action of the working beam. 

588. To describe the action of this engine, let us suppose 
that the piston c is drawn up to the top of the cylinder, by 
the weight of the pump rod g , as represented in the figure; 
that the cylinder itself is filled with steam, and that the stop¬ 
cock of the steam pipe is turned so that no more steam is ad¬ 
mitted. The cylinder was surrounded by another circular 
vessel, leaving a space between the two, into which the cold 
water was admitted. Suppose the cold water to be drawn 
by the condensing pipe i into this space, and consequently 
the steam to be condensed, leaving a vacuum within the cyl¬ 
inder. The consequence would be, that the pressure of the 


Describe the several parts of this engine. Describe the action of this #n 
gine. 


14 



























158 


STEAM ENGINE. 


atmosphere on the piston would instantly force it down to 
the bottom of the cylinder. This would give action to the 
pump k , by which a quantity of water would be drawn up 
from the well. 

589. Now the piston being forced to the bottom of the cyl¬ 
inder by the pressure of the atmosphere, unless relieved from 
that pressure, would not rise again, and therefore a quantity 
of steam must be admitted under it by the pipe b , so as to 
balance the pressure on the upper side. When this is effected, 
the piston is immediately drawn again to the top of the cyl¬ 
inder by the weight of the pump rod, and thus the several 
parts of the engine become in the precise position that they 
were when our description began ; and in order again to de¬ 
press the piston, a vacuum must once more be produced by 
the admission of cold water on the cylinder, and so on con¬ 
tinually. 

The power of these engines, although operating by the 
pressure of the atmosphere alone, was much greater than 
might at first be supposed. 

590. The -pressure of the atmosphere, when operating on 
a perfect vacuum, as we have already shown, amounts to 15 
pounds on every square inch of surface. The power of this 
engine therefore depended entirely on the number of square 
inches which the piston presented to this pressure. 

591. Now the number of square inches in a circle may be 
very nearly found by the following rule : 

Multiply the number of inches in the diameter by itself: divide 
the product by 14, and multiply the quotient thus obtained by 
11, and the result will be the number of square inches in the 
circle . 

592. Thus a piston having a diameter of only 13 inches, 
would be pressed down by a weight equal to 1980 pounds, 
or nearly one ton; and a piston twice this diameter, or 26 
inches, would be acted upon by a weight equal to 7920 
pounds, or nearly four tons. These estimates are, however, 
too high for practical results, for, after allowing for the fric¬ 
tion of the piston, and the imperfection of the vacuum, it was 
found, in practice, that only about 11 pounds of force to the 
square inch could actually be obtained. 


What is said of the power of these engines ? How may the number of 
square inches in a circle be found ? What would be the amount of pressure 
on a piston of 13 inches in diameter? What would be the pressure on a pis¬ 
ton of 26 inches in diameter? How much must be allowed for friction and 
imperfection of vacuum ? 



STEAM ENGINE. 


159 


593. Soon after the construction of these engines, an acci* 
dental circumstance suggested to the inventor a much better 
method of condensation than the effusion of cold water on 
the cylinder, which, as we have seen, was that first prac¬ 
ticed. In order to keep the piston air-tight, it was necessary 
to ha^e a quantity of water on it, which was supplied from 
a pipe placed over it. On one occasion, a piston was ob¬ 
served to descend several times with unusual rapidity, and 
this without waiting for the usual supply of condensing wa¬ 
ter. On examination, it was found that an aperture through 
the piston admitted the cold water directly to the steam in 
the cylinder, by which it was instantly condensed. 

594. On this suggestion, Newcomen abandoned his first 
method, and by the addition of a pipe, through which a jet 
of cold water was thrown into the cylinder, condensed the 
steam instantly, and much more perfectly than could be done 
even by waiting a long time for the gradual cooling of the cylin¬ 
der by the old method. This was a highly important improve¬ 
ment, and is substantially the method practiced to this day. 

595. Newcomen’s machine, though so imperfect, when 
compared with those of the present day, as hardly to deserve 
the name of a steam engine, was extensively employed in 
draining the English mines, and for nearly half a century 
was the only machine moved by the application of steam. 
And notwithstanding its material and obvious imperfections, 
still it must be considered as a lasting monument of the 
combining and inventive powers of a man, who appears origin¬ 
ally to have had no advantages in life, above what his expe¬ 
rience and observations as a blacksmith gave him. 

596. Watt's Engine .—It does not appear that any consid¬ 
erable improvements were made on Newcomen’s steam ap¬ 
paratus, until the time when James Watt began his experi¬ 
ments and inventions in about 1763. 

Watt was born at Greenock, in Scotland, and pursued the 
business of a mathematical instrument maker in London. 
He was endowed with a mind of the highest order, both as 
a philosopher and inventor, as will be evinced by the new 
combinations, improvements, and inventions, which he ap¬ 
plied to nearly every part of the apparatus to which steam 
has been employed as a moving power. 

How did Newcomen discover an improved method of condensing steam? 
What is said of Newcomen’s invention on the whole? When did Watt be¬ 
gin his experiments ? What is said of Watt’s capacity? W’hat were among 
Hie first improvements of the steam engine ? What change must be made in 
Newcomen’s cylinder, in order to press down the piston with steam? 



100 


STEaM engine. 


Fig. 125. 


597. Some of his first improvements, or perhaps more 
properly, inventions, were a pump, for the removal of the air 
and water, which were accumulated by the condensation ol 
the steam—the application of melted wax, or tallow, instead 
of water, to lubricate the piston, and keep it air-tight, and the 
employment of steam above the piston, to press it do^n, in¬ 
stead of the atmosphere, as in Newcomen’s engine. 

For the latter purpose, it was necessary to close the top of 
the cylinder, and allow the piston-rod to play through a 
steam tight stuffing-box, as is done at the present time in all 
steam engines. 

598. This improvement 
is represented by fig. 125, 
where s is the steam pipe 
proceeding from the boiler, 
and by which steam is 
admitted to the cylinder. 

The piston h works air-tight 
in the cylinder g , the rod of 
of which passes air-tight 
through the stuffing-box *. 

The upper valve box a con¬ 
tains a single valve, which 
when open, admits the 
steam into the cylinder, and 
also into the pipe which 
connects this with the low¬ 
er valve box. The lower 
box contains two valves, b 
and c; the valve 6, when 
open, admits the steam to 
pass from the cylinder 
above the piston, by the 
connecting tube, to the 
cylinder below the piston ; the valve c, when open, admits the 
steam to pass from below the cylinder, down into the con¬ 
denser d. This steam entering the condenser, meets the jet 
of water through the valve d, where it is condensed. The 
valve e, opening outwards, permits any steam which is not 
condensed, together with such atmospheric air as is accumu¬ 
lated, to pass away. 

The valve a is called the upper steam valve ; b, the lower 



Wbat are the situations, names, and uses, of the valves in fig. 125 . 




















STEAM ENGINE. 


161 


steam valve ; c, the exhausting valve , and d, the condensing 
valve. 

599. Now let us see in what manner this machine will 
produce the alternate ascent and descent of the piston. 

In the first place, all the air which fills the cylinder and 
tubes must be expelled. To do this, the valves a, h , and c , 
must be opened. The steam will pass through the pipe s 7 
into the upper part of the cylinder, and along the tube down 
through the valves b and c into the condenser d. After the 
steam ceases to be condensed by the cold of the apparatus, it 
will rush out, mixed with air, through the valve e, which 
opens outwards. 

600. The apparatus is thus filled with steam, and all the 
valves are now to be closed; but in a few minutes a vacuum 
will be formed in the condenser, by the cold water thrown 
into that vessel. 

The apparatus being in this state, let the upper steam 
valve «, the exhausting valve c, and the condensing valve <?, 
be opened. Steam will thus be admitted through a, to press 
upon the top of the piston, the steam being prevented from 
circulating below the piston, by the valve b being closed. 
But the steam below the piston will rush through the ex¬ 
hausting valve c, into the condenser, where a jet of cold 
water through the condensing valve d , will instantly con¬ 
dense it, and thus leave a vacuum below the piston in the 
cylinder. Into this vacuum the piston is instantly pressed 
by the action of the steam in the upper part of the cylinder. 

601. When the piston has thus been forced to the bottom 
of the cylinder, let the valves a, c, and d , be closed, and let 
the lower steam valve b be opened. The effect of this will 
be, that the further ingress of steam will be stopped, and the 
further condensation of steam will cease, and thus the steam 
which is shut up within the apparatus, will press equally on 
all sides, so that the pressure on the upper and under sides 
of the piston will be equal. Thus there is no force to restrain 
the piston at the bottom of the cylinder, except its weight, 
which is more than balanced by the weight of the pump-rod 
at the other end of the beam, and by the preponderance of 
which the piston rises, as in the atmospheric engine. 

602. When the piston has arrived to the top of the cylin¬ 
der, the valves a, c, and d, are again opened, when steam 
again presses on the top of the piston, while a vacuum is 


Explain the manner in which this engine acts by means of the figure 


14* 



162 


STEAM ENGINE. 


formed below it, into which the piston is driven, as already 
shown, and so on continually. 

The valves of this engine were opened and closed by lev¬ 
ers, which were worked by the movement of the machinery. 
These, being unnecessary to explain the principle, are not 
shown in the drawing. 

603. Mr. Watt called this his single acting engine , because 
the steam acted only above the piston, and for the purpose 
of distinguishing it from his double acting engine , in which 
the piston was moved in both directions, by the force of steam. 

604. Double Acting Steam Engine. —After the construc¬ 
tion of the steam engine above described, Mr. Watt contin¬ 
ued his improvements and inventions, which resulted in the 
production of his double acting engine . This consisted in 
changing the steam alternately from below, to above the pis¬ 
ton, and at the same time forming a vacuum alternately in 
each end of the cylinder, into which the piston was forced. 
Thus the piston being at the top of the cylinder, steam was 
introduced from the boiler above it, while the steam in the 
cylinder below it was condensed. The piston was therefore 
pressed by the steam above it into a vacuum below. Hav¬ 
ing arrived at the bottom of the cylinder, the steam was 
changed in its direction, and sent below the piston, while a 
communication was formed between the upper part of the 
cylinder and the condenser, and thus a vacuum was formed 
above the piston, into which it was forced by the steam acting 
below it. In this manner was the piston moved by alter¬ 
nately substituting steam for a vacuum, and a vacuum for 
steam, on each side of the piston. 

605. Circular Motion of Machinery by means of Steam .— 
The action of the atmospheric engine of Newcomen, and of 
the improved, or single acting one of W att, was such as 
could not be applied to the continued motion of machinery. 
Their motions were well calculated to raise water from the 
mines by pumping, and for this purpose they were chiefly 
employed. Nor could these engines give a perpetual cir¬ 
cular motion, without some changes in their action, and ad- - 
ditions to their machinery. It is obvious, that the extended 
use of steam in driving machinery, absolutely required such 
a motion, and it appears that the genius of W att, soon after 
his experiments commenced, saw the vast consequences of 
such an application of this power, and he applied himself to 
the invention of machinery for this purpose accordingly. 

Why does Mr. Watt call this his single acting engine ? Describe Watt a 
double acting steam engine. 



STEAM ENGINE. 


103 


606. In Newcomen’s and Watt’s first engines, tlie end 
of the beam opposite to the piston could only be employed 
in lifting , since the power was applied only to force the 
piston downwards. But in the double acting engine, the 
power of steam was applied to the piston in both directions, 
and hence the opposite end of the beam had a force down¬ 
ward, as well as upward. If, therefore, instead of chains, 
rods of iron were attached to each arch head of the beam, 
the one rod connected with the piston, and the other with the 
machinery to be moved, it is plain that since the end of the 
beam, connected with the piston, would be pushed up and 
drawn down with a force equal to the power of the steam 
applied, the other end of the beam would act with equal 
force, and thus that a sufficient power might be obtained in 
both directions. 

607. The question with respect to the means by which a 
continued circular motion might be obtained from the alter¬ 
nate motion of the working end of the beam, did not remain 
long unsettled in the fertile mind of W att. A crank con¬ 
nected with the end of the beam by an inflexible or metallic 
rod, would convert its up and down motion into one of at 
least partial rotation. 

608. But still there remained a difficulty to be overcome 
with respect to the rotation of a- crank, for there are two 
positions in which the vertical motions of the working rod 
could give it no motion whatever. These are, when the 
axis of the crank a , fig. 126, 
the joint of the crank b , and 
the working rod, or connector, 
with the working beam c. are 
in the same right line as shown 
in the figure. In this case it 
is plain, that the vertical action 
of c could not move the crank 
in any direction. Again, 
when the joint b is turned 
down to d , so as to bring the 
working rod c, directly over 
the crank, it will be obvious 
that the upward or downward 
force of the beam, could not 
give a any motion whatever. 

Hence, in these two posi¬ 
tions the engine could have no 


Fig. 126. 





164 


STEAM ENGINE. 


effect in turning the crank, and, therefore, twice in every 
revolution, unless some remedy could be found for this defect, 
the whole machine must cease to act. 

609. Now, under Inertia, (21) we have shown that bod¬ 
ies, when once put in motion, have a tendency to continue 
that motion, and will do so, unless stopped by some opposing 
force. With respect to circular motion, this subject is suffi¬ 
ciently illustrated by the turning of a coach wheel on its axis 
when the tire is raised from the ground. Every one knows 
that when a wheel is set in motion, under such circumstan¬ 
ces, it will continue to revolve by its own inertia for some 
time, without any new impulse. 

610. This principle Watt applied to continue the motion 
of the crank. A large heavy iron wheel was fixed to the 
axis of the crank, which wheel being put in motion by the 
machinery, had the effect to turn the crank beyond the 
position in which we have shown the working rod had no 
power to move it, and thus enabled the working rod to con¬ 
tinue the rotation. 

611. Such a wheel, called the fly wheel, or balance 
wheel, is represented attached to the crank in fig. 126, and 
is now universally employed in all steam engines used in 
driving machinery. 

612. Governor, or Regulator .—In the application of steam 
to machinery for various purposes, a steady or equal motion 
is highly important; and although the fly wheel, just de¬ 
scribed, had the effect to equalize the motion of the engine 
when the power and the resistance were the same, yet when 
the steam was increased, or the resistance diminished or 
increased, there was no longer a uniform velocity in the 
working part of the engine. 

In order to remedy this defect, Mr. Watt applied to his 
engines an apparatus called a governor , and by which the 
quantity of steam admitted to the cylinder was so regulated 
as to keep the velocity of the engine nearly the same at all 
times. 

613. Of all the contrivances for regulating the motion of 
machinery, this is said to be the most effectual. It will be 
readily understood by the following description of fig. 127. 


What is said of the action of Newcomen’s and Watt’s first engine? Why 
were not their motions applicable to machinery ? Explain the reason why 
Watt’s double acting engine was applicable to the rotation of machinery, while 
his other engine was not. Explain the reason why a crank motion alone can 
not be converted into a continued rotation. 




STEAM ENGINE. 


165 



It consists of two heavy 
iron balls b , attached to 
the extremities of the two 
rods, bj e. These rods 
play on a joint at e, pass¬ 
ing through a mortice in 
the vertical stem d, d. 

At /, these pieces are uni¬ 
ted, by joints to the two 
short rods,/ h, which, at 
their upper ends, are again 

connected by joints at h , to a ring which slides upon the 
vertical stem d d. Now it will be apparent that when these 
balls are thrown outward, the lower links connected at f will 
be made to diverge, in consequence of which the upper links 
will be drawn down the ring with which they are connected 
at h. With this ring at i is connected a lever having its axis 
at g and to the other extremity of which, at k , is fastened a 
vertical piece, which is connected by a joint to the valve v. 
To the lower part of the vertical spindle d, is attached a 
grooved wheel w, around which a strap passes, which is 
connected with the axis of the fly wheel. 

614. Now when it so happens that the quantity of steam 
is too great, the motion of the fly wheel will give a propor¬ 
tionate velocity to the spindle d, d, by means of the strap 
around w, and by which the balls, by their centrifugal force, 
will be widely separated; in consequence of which the ring 
h will be drawn down. This will elevate the arm of the 
lever k , and by which the end i, of the short lever, connected 
with the valve v, in the steam pipe, will be raised, and thus 
the valve turned so as to diminish the quantity of steam ad¬ 
mitted to the piston. When the motion of the engine is 
slow, a contrary effect will be produced, and the valve turned 
so that more steam will be admitted to the engine. 

615. Low pressure Engine. —To comprehend the working 
of the piston, which is usually hid from the eye of the ob¬ 
server, it is only necessary to remember, that in the upper 
valve box there are two valves, called the upper steam valve , 
and the upper exhausting valve , and that in the lower steam 
box, or bottom of the cylinder, there are also two valves, 
called the lower steam valve, and the lower exhausting valve 


In what manner was the crank motion converted into one of perpetual rota 
tion ? Give a general description of the governor, by means of the figure 
What are the valves called in the upper, and what in the lower valve box ? 




160 


STEAM ENGINE. 


616. Now suppose the piston to be at the top of the cylin 
der, the cylinder below it being filled with steam, which ha? 
just pressed the piston up. Then let the upper steam valve , 
and the lower exhausting valve be opened, the other two being 
closed; the steam which fills the cylinder below the piston, 
will thus be allowed to pass through the exhausting valve 
into the condenser, and a vacuum will be formed below the 
piston. At the same time, the upper steam valve being open, 
steam will be admitted above the piston to press it down into 
the vacuum, which has been formed below. On the arrival 
of the piston to the bottom of the cylinder, the upper steam 
valve, and the lower exhausting valve are closed, and the 
lower steam valve, and upper exhausting valve are opened, on 
which the steam above the piston is condensed, while steam 
is admitted below the piston to press it into the vacuum thus 
formed, and so on continually. 

617. The upper steam valve, and lower exhausting valve 
are opened at the same time; the same being the case with 
the lower steam valve, and upper exhausting valve. 

618. This will be understood by the following description 
of fig. 128, which represents the essential parts of a steam 
engine of the present day. 

Fig. 128. 



619. 1st. The cylinder , in which the piston P is moved up 
or down, as the steam enters above or below it. 2d. The 


When the piston is at the top of the cylinder, what valves are opened ? 
When at the bottom, what valves are opened ? 





























STEAM ENGINE. 


167 


exhausting- and steam valves are placed above d and below c, 
but being within the tube cannot be seen. 3d. The steam, 
pipe e, which conveys the steam from the boiler, (not shown 
in the figure,) to the cylinder. 4th. The condenser C, for low 
pressure engines. 5th. The eduction pipe , which leads from 
the cylinder to the condenser. 6th. The hot water pump, k , 
which removes the condensed steam as it comes from the 
cylinder, h being its piston rod. 7th. The working beam a g, 
turning on its axis at i , and connected with the piston rod o , 
at one end, and with the crank of the^/Zy wheel m , by the rod 
g , Z, at the other. 8th. The fly wheel in , which gives con¬ 
tinuous motion to the whole machine, as already explained. 
It will be understood that this is the low pressure engine. 

There are several other parts of minor consequence to the 
steam engine, but it was thought that by introducing them, 
our figure would have become so complex that none of it 
could be understood, hence they are omitted. 

620. High Pressure Engine .—In the high pressure engines, 
the piston is pressed up and down by the force of the steam 
alone, and without the assistance of a vacuum. The addi¬ 
tional power of steam required for this purpose is very con¬ 
siderable, being equal to the entire pressure of the atmos¬ 
phere on the surface of the piston. W e have already had 
occasion to show that on a piston of 13 inches in diameter, 
the pressure of the atmosphere amounts to nearly two tons. 

621. Now in the low pressure engine, in which a vacuum 
is formed on one side of the piston, tITe force of steam re¬ 
quired to move it is diminished by the amount of atmospheric 
pressure equal to the size of the piston. 

622. But in the high pressure engine, the piston works in 
both directions against the weight of the atmosphere, and 
hence requires an additional power of steam equal to the 
weight of the atmosphere on the piston. 

623. These engines are, however, much more simple and 
cheap than the low pressure, since the condenser, cold water 
pump, air pump, and cold water cistern, are dispensed with ; 
nothing more being necessary than the boiler, cylinder, pis¬ 
ton, and valves. Hence for rail-roads, and all locomotive 
purposes, the high pressure engines are, and must be used. 

624. With respect to engines used on board of steam¬ 
boats, the low pressure are universally employed by the 


What constitutes a low pressure engine ? How much more force of steam 
is required in high than in low pressure engines ? What parts are dispensed 
with in high pressure engines ? 




168 


HORSE POWER. 


English, and it is well known, that few accidents from the 
bursting of machinery have ever happened in that country. 
In most of their boats two engines are used, each of which 
turns a crank, and thus the necessity of a fly wheel is 
avoided. 

625. In this country high pressure engines are in common 
use for boats, though they are not universally employed. In 
some, two engines are worked, and the fly wheel dispensed 
with, as in England. 

626. Accidents .—The great number of accidents which have 
happened in this country, whether on board of low or high pres¬ 
sure boats, must be attributed, in a great measure, to the eager¬ 
ness of our countrymen to be transported from place to place 
with the greatest possible speed, all thoughts of safety being 
absorbed in this passion. It is, however, true, from the very 
nature of the case, that there is far greater danger from the 
bursting of the machinery in the high, than in the low pres¬ 
sure engines, since not only the cylinder, but the boiler and 
steam pipes, must sustain a much higher pressure in order to 
gain the same speed, other circumstances being equal. 

HORSE POWER. 

627. When steam engines were first introduced, they were 
employed to work pumps for draining the English coal mines, 
thus taking the places of horses, which from the earliest 
times of using coal had performed this service. 

628. It being therefore already known how many horses 
were required to raise a certain amount of coal from a given 
depth, the powers of these engines were very naturally com¬ 
pared to those of horses, and thus an engine which would 
perform the work of ten horses, was called an engine of ten 
horse power. To this day the same term is used, with the 
same meaning, though very few appear to know either the 
origin of the term, or the amount of power it implies. 

629. Several engineers, after the term was thus used, made 
experiments, for the purpose of ascertaining the average 
strength of horses, with a view of fixing a standard of me-' 
chanical force which should be indicated by the term horse 
power. 

This was done by means which it is not necessarv >re 
to describe. 

Smeaton, a celebrated mechanical philosopher, estimated 


Where did steam engines first*take the place of horses ? What is the origin 
cf the term horse power? 



HORSE POWER. 


169 


that the average power of the horse, working eight hours a 
day, was equal to the raising of 23,000 pounds at the rate 
of one foot per minute. 

630. Messrs. Bolton and Watt caused experiments to be 
made with the horses used in the breweries of London, said 
to be the strongest in the world, and from the result they es¬ 
timated that 33,000 pounds raised at the rate of one foot per 
minute, was the value of a horse’s power, and this is the esti¬ 
mate now generally adopted. When, therefore, an engine 
is said to be so many horses’ power, it is meant that it is ca¬ 
pable of overcoming a resistance equal to so many times 
33,000 pounds raised at the rate of one foot per minute. 
Thus an engine of ten horse power is one capable of raising 
a load of 330,000 pounds one foot per minute, and so at this 
rate, whether the power be more or less. 

631. Power of Steam .—Experiment has proved that an 
ounce of water converted into steam will raise a weight of 
2,160 pounds one foot. A cubic foot of water contains 1,728 
cubic inches, and the power, therefore, of a cubic foot of 
water, when converted into steam will be equal to 2,160 mul¬ 
tiplied by 1,728, equal to 3,732,480 pounds. This, then, ex¬ 
presses the number of pounds weight which a cubic foot of 
water would raise one foot when converted into steam, sup¬ 
posing that its entire mechanical force could be rendered 
available. But in practice it is estimated that the friction, and 
weight of the machinery in action, requires about four-tenths 
of the whole force, while six-tenths only remain as an actual 
mechanical power. 

632. Quantity of water required for each Horse Power .—One 
horse power, as already explained, is equal to a force which 
will raise 33,000 pounds one foot high per minute. This 
being multiplied by 60 will show the force required to raise 
the same weight at the rate of one foot per hour, namely, 
33,000x60 = 1,980,000 pounds. 

633. Now the quantity of water required for this effect, 
will be found by considering, as already shown, that a cubic 
inch of water in the form of steam, is equal to a force raising 
2,160 pounds a foot. If we divide 1,980,000, therefore, by 


What was Smeaton’s estimate of a horse’s power ? WTiat was Watt and 
Bolton’s estimate of a horse’s power? What is meant by a horse’s power at 
the present time ? How many horses would raise 33,000 pounds on< foot per 
minute? What is the power of a square inch of water converted in o steam? 
What is the power of a cubic foot of water converted into steam? t ow much 
power is lost in acting upon the engine ? How many cubic inches of water is 
required to produce a one horse power ? 




170 


HORSE POWER. 


2.160, we shall have the number of cubic inches of water 
required to produce a one horse power, namely, 9,160. But 
we have already shown that only 6 parts out of 10 of the 
force of steam can be calculated on as a moving power, 4 
parts being expended on the action of the engine. To find, 
then, the amount of waste in 916 cubic inches of water, we 
must divide that number by 6, and multiply the result by 4, 
when we shall have 610 as the number of cubic inches of 
water wasted. The total quantity of water, therefore, which 
is turned into steam per hour, to produce a one horse power, 
is equal to 610 added to 916, namely, 1,526 cubic inches. 
Hence we see the necessity of the immense capacities of the 
boilers of large steamboats. 

634. Amount of Mechanical Virtue in Coal .—For more 
than thirty years the engineers of many of the English coal 
mines have published annual accounts of their experiments 
with the steam engines under their care, for the purpose of 
ascertaining the exact amount of coal required to perform 
certain duties. The result of these experiments are among 
the most curious and instructive facts which the lights of 
science at the present day have thrown upon the manufac¬ 
turing arts. They were entirely unexpected to the owners 
of the mines, and equally so to men of science. 

635. In the report of the engineers thus employed, for 
1835, it was announced that a steam engine employed at a 
copper mine in Cornwall, had raised, as its average work, 
95 millions of pounds a foot high, with a single bushel of bi¬ 
tuminous coal. 

This mechanical effect was so enormous and so unex¬ 
pected, that the best judges of the subject considered it be¬ 
yond the bounds of credulity; the proprietors, therefore, 
agreed that another trial should be made in the presence of 
competent witnesses: when, to the astonishment of all, the 
result exceeded the former report by 30 millions of pounds. 
In this experiment, for every bushel of coal consumed under 
the boiler, the engine raised 1251 millions of pounds one foot 
high. 

636. On this subject, Dr. Lardner, in his treatise on the 
steam engine, has made the following calculations: 

A bushel of coal weighs 84 pounds, and can lift 56,027 
tons a foot high, therefore, a pound of coal would raise 667 

How do you find how many cubic inches of water there is in a one horse 
power ? What amount of weight is it said a bushel of coal will raise by means 
of steam? What was the weight raised by the second trial? What weight 
will a pound of coal raise ? 6 




ACOUSTICS. 171 

tons to the same height; and an ounce would raise 42 tons 
one foot high, or it would lift 18 pounds a mile high. 

Since a force of 18 pounds is capable of drawing two tons 
upon a rail-way, it follows that an ounce of coal would draw 
2 tons a mile, or 1 ton two miles. [In the common engines, 
however, the actual consumption of coal is equal to about 8 
ounces per ton for every mile.] 

637. The great Egyptian pyramid has a base of 700 feet 
each way, and is 500 feet high; its weight amounting to 
12,760,000,000 pounds. To construct it, is said to have cost 
the labor of 100,000 men for 20 years. Yet according to 
the above calculations, its materials could have been raised 
from the ground to their present positions by the combustion 
of 479 tons of coal. 


ACOUSTICS. 

638. Acoustics is that branch of natural philosophy which 
treats of the origin , propagation, and effects of sound. 

639. When a sonorous, or sounding body is struck, it is 
thrown into a tremulous, or vibrating motion. This motion 
is communicated to the air which surrounds us, and by the 
air is conveyed to our ear drums, which also undergo a 
vibratory motion, and this last motion, throwing the audi¬ 
tory nerves into action, we thereby gain the sensation of 
sound. 

640. If any sounding body, of considerable size, is sus¬ 
pended in the air and struck, this tremulous motion is dis¬ 
tinctly visible to the eye, and while the eye perceives its mo¬ 
tion, the ear perceives the sound. 

641. Proof by the Air Pump. —That sound is conveyed to 
the ear by the motion which the sounding body communi¬ 
cates to the air, is proved by an interesting experiment with 
the air pump. Among philosophical instruments, there is a 
small bell, the hammer of which is moved by a spring con¬ 
nected with clock-work, and which is made expressly for 
this experiment. 

How great a force may an ounce of coal be made to produce ? What is the 
size and weight of the great pyramid of Egypt ? What weight of coal would 
be required to raise its materials to their present elevation ? What is acous¬ 
tics ? When a sonorous body is struck within hearing, in what manner do we 
gain from it the sensation of sound ? How is it proved that sound is conveyed 
to the ear by the medium of the air ? 




172 


ACOUSTICS. 


If this instrument be wound up, and placed under the re¬ 
ceiver of an air pump, the sound of the bell may at first be 
heard to a considerable distance, but as the air is exhausted, 
it becomes less and less audible, until no longer to be heard, 
the strokes of the hammer, though seen by the eye, produ¬ 
cing no effect upon the ear. Upon allowing the air to re¬ 
turn gradually, a faint sound is at first heard, which becomes 
louder and louder, until as much air is admitted as was 
withdrawn. 

642. Diving Bell .—On the contrary, when the air is more 
dense than ordinary, or when a greater quantity is contained 
in a vessel, than in the same space in the open air, the effect 
of sound on the ear is increased. This is illustrated by the 
use of the diving bell. 

The diving bell is a large vessel, open at the bottom, un¬ 
der which men descend to the beds of rivers, for the purpose 
of obtaining articles from the wrecks of vessels. When 
this machine is sunk to any considerable depth, the water 
above, by its pressure, condenses the air under it with great 
force. In this situation, a whisper is as loud as a common 
voice in the open air, and an ordinary voice becomes painful 
to the ear. 

643. Again, on the tops of high mountains where the pres¬ 
sure, or density of the air is much less than on the surface 
of the earth, the report of a pistol is heard only a few rods, 
and the human voice is so weak as to be inaudible at ordi¬ 
nary distances. 

Thus, the atmosphere which surrounds us, is the medium 
by which sounds are conveyed to our ears, and to its vibra¬ 
tions we are indebted for the sense of hearing, as well as to 
all we enjoy from the charms of music. 

644. Solids Conduct Sound .—The atmosphere, though 
the most common, is not, however, the only, or the best con¬ 
ductor of sound. Solid bodies conduct sound better than 
elastic fluids. Hence, if a person lay his ear on a long stick 
of timber, the scratch of a pin may be heard from the other 
end, which could not be perceived through the air. 

645. The earth conducts loud rumbling sounds made 
below its surface to great distances. Thus, it is said, that 
in countries where volcanoes exist, the rumbling noise 


When the air is more dense than ordinary how does it affect sound ? What 
is said of the effects of sound on the tops of high mountains ? Which are the 
best conductors of sound, solid or elastic substances ? What is said of the 
earth as a conductor of sounds ? 



ACOUSTICS. 


173 

which generally precedes an eruption, is heard first by the 
beasts of the field, because their ears are commonly near the 
ground, and that by their agitation and alarm, they give 
warning of its approach to the inhabitants. 

The Indians of our country, by laying their ears on the 
ground, Will discover the approach of horses or men when 
they are at such distances as not to be heard in any other 
manner. 

646. Velocity of Sound .—Sound is propagated through 
the air at the rate of 1142 feet in a second of time. When 
compared with the velocity of light, it therefore moves but 
slowly. Any one may be convinced of this by watching the 
discharge of cannon at a distance. The flash is seen appa¬ 
rently at the instant the gunner touches fire to the powder; 
the whizzing of the ball, if the ear is in its direction, is next 
heard, and lastly, the report. 

Biot's Experiment .—Solid substances convey sounds with 
greater velocity than air, as is proved by the following 
experiment, lately made at Paris, by M. Biot. 

647. At the extremity of a cylindrical tube, upwards of 
3000 feet long, a ring of metal was placed, of the same 
diameter as the aperture of the tube; and in the centre of 
this ring, in the mouth of the tube, was suspended a clock 
bell and hammer. The hammer was made to strike the 
ring and the bell at the same instant, so that the sound of the 
ring would be transmitted to the remote end of the tube, 
through the conducting power of the tube itself, while the 
sound of the bell would be transmitted through the medium 
of the air inclosed in the tube. The ear being then placed 
at the remote end of the tube, the sound of the ring, trans¬ 
mitted by the metal of the tube, was first heard distinctly, 
and after a short interval had elapsed, the sound of the bell, 
transmitted by the air in the tube, was heard. The result 
of several experiments was, that the metal conducted the 
sound at the rate of about 11,865 feet per second, which is 
about ten and a half times the velocity with which it is con¬ 
ducted by the air. 

648. Sound moves forward in straight lines, and in this 
respect follows the same laws as moving bodies, and light. 


How is it said that the Indians discover the approach of horses ? How fast 
does sound pass through the air? Which convey sounds with the greatest 
velocity, solid substances or air? Describe the experiment, proving that 
sound is conducted by a metal with greater velocity than by the nr. In what 
lines does sound move? 

15* 



174 


ACOUSTICS. 


Fig. 129. 


11 


Fig. 130. 


It also follows the same laws in being reflected, or thrown 
back, when it strikes a solid, or reflecting surface. 

649. Echo .—If the surface be smooth, and of considerable 
dimensions, the sound will be reflected, and an echo will be 
heard ; but if the surface is very irregular, soft, or small, no 
such effect will be produced. 

In order to hear the echo, the ear must be placed in a 
certain direction, in respect to the point where the sound is 
produced, and the reflecting surface. 

If a sound be produced at a, fig. 129, 
and strike the plain surface 6, it will be 
reflected back in the same line, and the 
echo will be heard at c or a. That is, 
the angle under which it approaches 
the reflecting surface, and that under 
which it leaves it, will be equal. 

650. Whether the sound strikes the 
reflecting surface at right-angles, or 
obliquely, the angle of approach, and 
the angle of reflection, will always be 
the same, and equal. 

This is illustrated 
by fig. 130, where 
suppose a pistol to be 
fired at a, while the 
reflecting surface is 
at c; then the echo 
will be heard at 6, the 
angles 2 and 1 being 
equal to each other. 

651. Reverberation of Sound .—If a sound be emitted be¬ 
tween two reflecting surfaces, parallel to each other, it will 
reverberate, or be answered backwards and forwards several 
times. 

Thus, if the sound be made at a , fig. 

131, it will not only rebound back 
again to a, but will also be reflected 
from the points c and d, and were such 
reflecting surfaces placed at every point 
around a circle from a, the sound would 
be thrown back from them all, at the 
same instant, and would meet again at 
the point a. 

* We shall see, under the article Optics, 



Fig. 131. 











ACOUSTICS. 


Fig. 132. 

e 


175 

that light observes exactly the same law in respect to its 
reflection from plane surfaces, and that the angle at which 
it strikes, is called the angle of incidence , and that under 
which it leaves the reflecting surface, is called the angle 
of reflection. The same terms are employed in respect to 
sound. 

652. Reflection in a Circle. —In a circle, sound is reflected 
from every plane surface placed around it, and hence, if the 
sound is emitted from the centre of a circle, this centre will 
be the point at which the echo will be most distinct. 

Suppose the ear to be placed 
at the point a, fig. 132, in the 
centre of a circle; and let a 
sound be produced at the same 
point, then it will move along 
the line a e, and be reflected from 
the plane surface, back on the 
same line to a; and this will 
take place from all the plane 
surfaces placed around the cir¬ 
cumference of a circle; and as 
all these surfaces are at the 
same distance from the centre, 

so the reflected sound will arrive at the point a, at the 
same instant; and the echo will be loud, in proportion to the 
number and perfection of these reflecting surfaces. 

653. Whispering Gallery .—It is apparent that the auditor, 
in this case, must be placed in the centre from which the 
sound proceeds, to receive the greatest effect. But if the 
shape of the room be oval, or elliptical, the sound may be 
made in one part, and the echo will be heard in another part, 
because the ellipse has two points, called foci, at one of 
which, the sound being produced, it will be concentrated in 
the other. 

Suppose a sound to be proc uced at a, fig. 133, it will be reflect- 



From what kind of surface is sound reflected, so as to produce an echo T 
Explain fig. 129. Explain fig. 130, and show in what direction sound approaches 
and leaves a reflecting surface. What is the angle under which sound strikes 
a reflecting surface, called ? What is the angle under which it leaves a reflect¬ 
ing surface called ? Is there any difference in the quantity of these two an¬ 
gles ? Suppose a pistol to be fired in the centre of a circular room, where 
would be the echo? Explain fig. 131, and give the reason. Suppose a sound 
to be produced in one of the foci of an ellipse, where then might it be most dis¬ 
tinctly heard ? 











176 


ACOUSTICS. 


ed from the sides of the room, the an- Fi s- 133 - 

gles of incidence being equal to those 
of reflection, and will be concentra¬ 
ted at b. Hence, a hearer standing 
at b, will be affected by the united 
rays of sound from different parts of 
the room, so that a whisper at a, 
will become audible at b , when it 
would not be heard in any other 
part of the room. Were the sides 
of the room lined with a polished 
metal, the rays of light or heat 
would be concentrated in the same 
manner. 

The reason of this will be understood, when we consider 
that an ear, placed at c, will receive only one ray of the 
sound proceeding from a, while if placed at. b, it will receive 
the rays from all parts of the room. Such a room, whether 
constructed by design or accident, would be a whispering gal- 



654. On a smooth surface, the rays, or pulses of sound, 
will pass with less impediment than on a rough one. For 
this reason, persons can talk to each other on the opposite 
sides of a river, when they could not be understood at the 
same distance over the land. The report of a cannon at sea, 
when the water is smooth, may be heard at a great distance, 
but if the sea is rough, even without wind, the sound will be 
broken, and will reach only half as far. 

655. Musical Instruments. — The strings of musical instru¬ 
ments are elastic cords , which being fixed at each end, produce 
sounds by vibrating in the middle. 

The string of a violin ox piano, when pulled to one side by 
its middle, and let go, vibrates backwards and forwards, like 
a pendulum, and striking rapidly against the air, produces 
tones, which are grave, or acute, according to its tension, 
size, or length. 

656. The man- Fig- 134. 

ner in which C 

such a string vi¬ 
brates, is shown 
by fig. 134. 

If pulled from 
e to a, it will not 
stoj again at e. 



4 










ACOUSTICS. 


177 

but in passing from a to e, it will gain a momentum, which 
will carry it to c, and in returning, its momentum will again 
carry it to d , and so on, backwards and forwards, like a pen¬ 
dulum, until its tension, and the resistance of the air, will 
finally bring it to rest. 

The grave, or sharp tones of the same string, depend on 
its different degrees of tension; hence, if a string be struck, 
and while vibrating, its tension be increased, its tone will be 
changed from a lower to a higher pitch. 

657. Strings of the same length are made to vibrate slow, 
or quick, and consequently to produce a variety of sounds, 
by making some larger than others, and giving them differ¬ 
ent degrees of tension. The violin and bass viol are familiar 
examples of this. The low, or bass strings, are covered with 
metallic wire, in order to make their magnitude and weight 
prevent their vibration from being too rapid, and thus they 
are made to give deep or grave tones. The other strings are 
diminished in thickness, and increased in tension, so as to 
make them produce a greater number of vibrations in a given 
time, and thus their tones become sharp or acute in propor¬ 
tion. 

658. Under certain circumstances, a long string will divide 
itself into halves, thirds, or quarters, without depressing any 
part of it, and thus give several harmonious tones at the 
same time. 

./Eolian* Harp. —The fairy tones of the iEolian harp are 
produced in this manner. This instrument consists of a sim¬ 
ple box of wood, with four or five strings, two or three feet 
long, fastened at each end. These are tuned in unison, so 
that when made to vibrate with force, they produce the same 
tones. But when suspended in a gentle breeze, each string, 
according to the manner or force in which it receives the 
blast, either sounds, as a whole, or is divided into several 
parts, as above described. “ The result of which,” says Dr. 
Arnot, “ is the production of the most pleasing combination 
and succession of sounds, that the ear ever listened to, or fan¬ 
cy perhaps conceived. After a pause, this fairy harp is often 
heard beginning with a low and solemn note, like the base 


Explain fig. 133, and give the reason. Why is it that persons can converse 
on the opposite sides of a river, when they could not hear each other at the 
same distance over the land ? How do the strings of musical instruments pro¬ 
duce sounds ? Explain fig. 134. On what do the grave or acute tones of the 
same string depend ! Why are the bass strings of instruments covered with 
metallic wire ? Why is there a variety of tones in the JEolian harp, since alt 
the strings are tuned in unison ? 



WIND. 


17* 

of distant music in the sky; the sound then swells as if ap¬ 
proaching, and other tones break forth, mingling with the 
first, and with each other.” 

659. The manner in which a string vibrates in parts, will 
be understood by fig. 135. 


Fig. 135. 



Suppose the whole length of the string to be from a to 5, 
and that it is fixed at these two points. The portion from 
b to c vibrates as though it was fixed at c, and its tone dif¬ 
fers from those of the other parts of the string. The same 
happens from c to eZ, and from d to a. While a string is 
thus vibrating, if a small piece of paper be laid on the part c, 
or d, it will remain, but if placed on any other part of the 
string, it will be shaken off. 

ATMOSPHERIC PHENOMENA. 

660. The term Atmosphere is from two Greek words, 
which signify vapor and sphere. It is the air which surrounds 
the earth to the height of 45 miles, and is essential to the lives 
of all animals, and the production of all vegetables. 

661. All meteorological phenomena, with which we are ac¬ 
quainted, depend chiefly, if not entirely on the influence ol 
the atmosphere. Fogs, winds, rain, dew, hail, snow, thun¬ 
der, lightning, electricity, sound, and a variety of other phe¬ 
nomena of daily occurrence belong to the atmosphere. We 
have, however, only room for the most common result of at¬ 
mospheric changes, Wind and Rain, 

WIND. 

662. Wind is nothing more than air in motion. The use 
of a fan, in warm weather, only serves to move the air, and 
thus to make a little breeze about the person using it. 

663. As a natural phenomenon, that motion of the air- 
which we call wind, is produced in consequence of there be¬ 
ing a greater degree of heat in one place than in another 


Explain fig. 135, showing the manner in which strings vibrate in parts. 
What is the atmosphere? How high does the atmosphere extend? What 
phenomena mentioned, depend on the atmosphere? What is wind? Asp 
natural phenomenon, how is wind produced, or, what is the cause of wind ? 







WIND. 


179 

The air thus heated, rises upward, while that which sur¬ 
rounds this, moves forward to restore equilibrium. 

The truth of this is illustrated by the fact, that during the 
burning of a house in a calm night, the motion of the air to¬ 
wards the place where it is thus rarefied, makes the wind 
blow from every point towards the flame. 

664. Sea, and Land Breeze .—In islands, situated in hot 
climates, this principle is charmingly illustrated. The land, 
during the day time, being under the rays of a tropical sun, 
becomes heated in a greater degree than the surrounding 
ocean, and, consequently, there rises from the land a stream 
of warm air, during the day, while the cooler air from the 
surface of the water, moving forward to supply this partial 
vacancy, produces a cool breeze setting inland on all sides of 
the island. This constitutes the sea breeze , which is so de¬ 
lightful to the inhabitants of those hot countries, and without 
which men could hardly exist in some of the most luxuriant 
islands between the tropics. 

During the night, the motion of the air is reversed, be¬ 
cause the earth being heated superficially, soon cools when 
the sun is absent, while the water being warmed several feet 
below its surface, retains its heat longer. 

Consequently, towards morning, the earth becomes colder 
than the water, and the air sinking down upon it, seeks an 
equilibrium, by flowing outwards, like rays from a centre, and 
thus the land breeze is produced. 

The wind then continues to blow from the land until tne 
equilibrium is restored, or until the morning sun makes the 
land of the same temperature as the water, when for a time 
there will be a dead calm. Then again the land becoming 
warmer than the water, the sea breeze returns as before, and 
thus the inhabitants of those sultry climates are constantly 
refreshed during the summer seasons, with alternate land 
and sea breezes. 

665. Trade Winds .—At the equator, which is a part of 
the earth continually under the heat of a burning sun, the 
air is expanded, and ascends upwards, so as to produce cur¬ 
rents from the north and south, which hnove forward to sup¬ 
ply the place of the heated air as it rises. These tw o cur¬ 
rents, coming from latitudes where the daily motion cf the 


How is this illustrated ? In the islands of hot climates, why does the wind 
blow inland during the day, and off the land during the night ? What are these 
breezes called ? What is said of the ascent of heated air at the equator T What 
is the consequence on the air towards the north and south . 





WIN0. 


180 

earth is less than at the equator, do not obtain its full rate oi 
motion, and therefore, when they approach the equator, do 
not move so fast eastward as that portion of the earth, by 
the difference between the equator’s velocity, and that of the 
latitudes from which they come. This wind therefore falls 
behind the earth in her diurnal motion, and consequently 
has a relative motion towards the west. This constant 
breeze towards the west is called the trade wind ) because a 
large portion of the commerce of nations comes within its 
influence. 

666. Counter Currents .—While the air in the lower re¬ 
gions of the atmosphere is thus constantly flowing from the 
north and south towards the equator, and forming the -trade 
winds between the tropics, the heated air from these regions 
as perpetually rises, and forms a counter current through the 
higher regions, towards the north and south from the tropics, 
thus restoring the equilibrium. 

667. This counter motion of the air in the upper and lower 
regions is illustrated by a very simple experiment. Open a 
door a few inches, leading into a heated room, and hold a 
lighted candle at the top of the passage; the current of air, 
as indicated by the direction of the flame, will be out of the 
room. Then set the candle on the floor, and it will show 
that the current is there into the room. Thus, while the 
heated air rises and passes out of the room, that which is 
colder flows in, along the floor, to take its place. 

This explains the reason why our feet are apt to suffer 
with the cold, in a room moderately heated, while the other 
parts of the body are comfortable. It also explains why 
those who sit in the gallery of a church are sufficiently 
warm, while those who sit below may be shivering with the 
cold. 

668. From such facts, showing the tendency of heated 
air to ascend, while that which is colder moves forward to 
supply its place, it is easy to account for the reason why the 
wind blows perpetually from the north and south towards 
the tropics ; for the air being heated, as stated above, it as ¬ 
cends, and then flows north and south towards the poles - 
until, growing cold, it sinks down and again flows towards 
the equator. 

669. Perhaps these opposite motions of the two currents 
will be better understood by the sketch, figure 136. 


How are the trade winds formed ? While the air in the lower regions flows 
from the north and south towards the equator, in what direction does it flow in 
aigher regions? 




RAIN 


is; 


Fig. 136. 
d e 



c a 


Suppose a b c to represent a portion of the earth’s surface, 
a being towards the north pole, c towards the south pole, and 
b the equator. The currents of air are supposed to pass in 
the direction of the arrows. The wind, therefore, from a to b 
would blow, on the surface of the earth, from north to south, 
while from e to «, the upper current would pass from south 
to north, until it came to a, when it would change its direc- 
tion towards the south. The currents in the southern hemis¬ 
phere being governed by the same laws, would assume simi¬ 
lar directions. 


RAIN. 

670. Rain is falling water in the form of drops. It appears 
to result from the meeting of two clouds of different tempera¬ 
tures. 

In explaining the theory of rain, it must be understood, 
that warm air has a greater capacity for moisture than cold. 
It is also ascertained, that the capacity increases at a much 
faster ratio than the increase of temperature itself, and hence 
it follows that if two clouds at different temperatures, com¬ 
pletely saturated, meet and mingle together, a precipitation of 
moisture must take place in consequence of the mixture. 
This would result from the fact that the warmest cloud con¬ 
tained a greater portion of moisture than indicated by its tem¬ 
perature, as stated above, while the mixture would form a 
mean temperature, but the mean quantity of vapor could not 
be retained, since the sum of their capacities for vapor would 
thus be diminished. 

671. Suppose for example, that at the temperature of 15 
degrees, air can hold 200 parts of moisture; then at 30 de- 


How is this counter current in lower and upper regions illustrated by a 
simple experiment? What common fact does this experiment illustrate? 
What is rain ? What is said of the ratio of capacity for moisture, increasing 
faster than the temperature in clouds ? Explain the reason why, when two 
olouds meet of different temperatures, rain is the result. 

16 



182 


OPTICS. 


grees it would hold 400 parts, and at 45 degrees 800 parts. 
Now let two equal bulks of this air, one at 15, and the other 
at 45 degrees be mixed, the compound would then contain 
200 and 800 parts of moisture—1000, that is, 500 each, and 
the temperature of the mixture would be 30 degrees. But 
at this temperature air is saturated with 400 parts, of vapor, 
therefore 100 parts is rejected and falls in the form of rain. 

This is Dr. Huttons’ theory of rain, and observation has 
seemed to prove its truth. 

672. Rain Gauge. —This is an instrument designed to 
measure the quantity of rain, which falls at any given time 
and place. 

A variety of forms, some quite complicated, 
have been invented for this purpose. The 
most simple and convenient, for common pur¬ 
poses, is that represented by fig. 137. It may 
be two feet high, round in form, and made of 
tin, or copper, well painted. It is furnished 
with a small metallic faucet for drawing off the 
water, and into the stem of this, is inserted a 
glass tube, as a scale, divided into inches and 
tenths of inches. This may be done by 
means of paper, pasted on and then varnished. 

The water will stand at the same height in 
the glass scale that it does in the cylinder, and 
outside the quantity may be known at a glance. If the fun¬ 
nel, or top, is twice the size of the cylinder, then, an inch in 
the scale will indicate half an inch received into the gauge, 
or these proportions may be a tenth, when much accuracy is 
required. 


Fig. 137. 




being on the 


OPTICS. 

673. Optics is that science which treats of vision, and the 
properties and phenomena of light. 

The term optics is derived from a Greek word, which sig¬ 
nifies seeing. 

This science involves some of the most elegant and im¬ 
portant branches of natural philosophy. It presents us with 


What is the design of the rain gauge ? What are the form and materials 
of this instrument? Describe the scale, and what it indicates with respect to 
the size of the funnel and cylinder? Define Optics. What is said of the ele 
gance and importance of this science ? 









OPTICS. 


183 


experiments which are attractive by their beauty, and which 
astonish us-by their novelty; and, at the same time, it inves¬ 
tigates the principles of some of the most useful among the 
articles of common life. 

674. There are two opinions concerning the nature of 
light. Some maintain that it is composed of material parti¬ 
cles, which are constantly thrown off from the luminous 
body; while others suppose that it is a fluid, diffused through 
all nature, and that the luminous, or burning body, occasions 
waves or undulations in this fluid, by which the light is 
propagated in the same manner as sound is conveyed through 
the air. The most probable opinion, however, is, that light 
is composed of exceedingly minute particles of matter. But 
whatever may be the nature or cause of light, it has certain 
general properties or effects which we can investigate. 
Thus, by experiment, we can determine the laws by which 
it is governed in its passage through different transparent sub¬ 
stances, and aieo those by which it is governed when it strikes 
a substance through which it cannot pass. We can like¬ 
wise test its nature to a certain degree, by decomposing or 
dividing it into its elementary parts, as the chemist decom¬ 
poses any substance he wishes to analyze. 

675. Definitions .—To understand the science of optics, it 
is necessary to define several terms, which, although some of 
them may be in common use, have a technical meaning, 
when applied to this science. 

a. Light is that principle, or substance, which enables us 
to see any body from which it proceeds. If a luminous sub¬ 
stance, as a burning candle, be carried into a dark room, the 
objects in the room become visible, because they reflect the 
light of the candle to our eyes. 

h. Luminous bodies are such as emit light from their own 
substance. The sun, fire, and phosphorus, are luminous 
bodies. The moon, and the other planets, are not luminous, 
since they borrow their light from the sun. 

c. Transparent bodies are such as permit the rays of light 
to pass freely through them. Air and some of the gases 
are perfectly transparent, since they transmit light without 
being visible themselves. Glass and water are also consid¬ 
ered transparent, but they are not perfectly so, since they are 


What are the two opinions concerning the nature of light ? What is the 
most probable opinion ? What is light? What is a luminous body ? What is 
a transparent body ? Are glass and water perfectly transparent ? How is it 
proved that air is perfectly transparent 7 



184 


OPTICS. 


themselves visible, and therefore do not suffer the light t6 
pass through them without interruption. 

d. Translucent bodies are such as permit the light to pass, 
but not in sufficient quantity to render objects distinct, when 
seen through them. 

e. Opaque is the reverse of transparent. Any body which 
permits none of the rays of light to pass through it, is 
opaque. 

f Illuminated , enlightened. Any thing is illuminated 
when the light shines upon it so as to make it visible. 
* Every object exposed to the sun is illuminated. A lamp 
illuminates a room, and every thing in it. 

g. A Ray is a single line of light, as it comes from a lu¬ 
minous body. 

h. A Beam of light is a body of parallel rays. 

i. A Pencil of light is a body of diverging or converging rays. 

k. Divergent rays, are such as come from a point, and con¬ 
tinually separate wider a part, as they proceed. 

l. Convergent rays, are those which approach each other, 
so as to meet at a common point. 

m. Luminous bodies emit rays, or pencils of light, in every 
direction, so that the space through which they are visible is 
filled with them at every possible point. 

676. Thus, the sun illuminates every point of space, with¬ 
in the whole solar system. A light, as that of a light-house, 
which can be seen from the distance of ten miles in one di¬ 
rection, fills every point in a circuit of ten miles from it, with 
light. Were this not the case, the light from it could not 
be seen from every point within that circumference. 

677. Motion of Light.—The rays of light move forward in 
straight lines from the luminous body, and are never turned out 
of their course, except by some obstacle. 

Let a, fig. Fig- 138. 

138,beabeam 
of light from 
the sun pass¬ 
ing through a 
small orifice 
in the win- 

What are translucent bodies ? What are opaque bodies ? What is meant 
by illuminated ? What is a ray of light ? What is a beam ? What a pencil ? 
W T hat. are divergent rays ? What are convergent rays ? In what direction do 
luminous bodies emit light ? How is it proved that a luminous body fills every 
point within a certain distance with light f Why cannot a beam of light be 
seen through a bent tube ? 




OPTICS. 


185 


dow shutter b. The sun cannot be seen through the crooked 
tube c, because the beam passing in a straight line, strikes 
the side of the tube, and therefore does not pass through it. 

678. All the illuminated bodies, whether natural or arti¬ 
ficial, throw off light in every direction of the same coloi as 
themselves, though the light with which they are illuminated 
is white or without color. 

This fact is obvious to all who are endowed with sight. 
Thus the light proceeding from grass is green, while that 
proceeding from a rose is red, and so of every other color. 

We shall be convinced, in another place, that the white 
light with which things are illuminated, is really composed 
of several colors, and that bodies reflect only the rays of their 
own color, while they absorb all the other rays. 

679. Velocity of Light. —Light moves with the amazing 
rapidity of about 95 millions of miles in 8| minutes, since it 
is proved by certain astronomical observations, that the light 
of the sun comes to the earth in that time. This velocity is 
so great, that to any distance at which an artificial light can 
be seen, it seems to be transmitted instantaneously. 

If a ton of gunpowder were exploded on the top of a 
mountain, where its light could be seen a hundred miles, no 
perceptible difference would be observed in the time of its 
appearance on the spot, and at the distance of a hundred 
miles. 

REFRACTION OF LIGHT. 

680. Although a ray of light will pass in a straight line y 
when not interrupted, yet when it passes obliquely from one 
transparent body into another , of a different density , it leaves 
its linear direction , and is bent, or refracted , more or less, out 
of its former course. 

This change in the direction of light, seems to arise from 
a certain power, or quality, which transparent bodies possess 
in different degrees; for some substances bend the rays of light 
much more obliquely than others. 

The manner in which the rays of 
light are refracted, may be reddily 
understood by fig. 139. 

Let a be a ray of the sun’s light, 
proceeding obliquely towards the 
surface of the water c, d, and let e 
be the point which it would strike, 
if moving only through the air. 

Now, instead of passing through 
16 * 






186 


OPTICS. 


the water in the line a, e, it will be bent or refracted, on en¬ 
tering the water, from o to n, and having passed through the 
fluid it is again refracted in a contrary direction on passing 
out of the water, and then proceeds onward in a straight 
line as before. 

681. Cup and Shilling. —The refraction of water is beau¬ 
tifully proved by the following simple experiment. Place an 
empty cup, fig. 140, with a shilling on the bottom, in such a 
position that the side of the cup will just hide the piece of 
money from the eye. Then 
let another person fill the 
cup with water, keeping 
the eye in the same posi¬ 
tion as before. As the 
water is poured in, the 
shilling will become visi¬ 
ble, appearing to rise with 
the water. The effect of 
the water is to bend the 
ray of light coming from 
the shilling, so as to make 
it meet the eye below the 
point where it otherwise would. Thus the eye could not 
see the shilling in the direction of c, since the line of vision 
is towards <z, and c is hidden by the side of the cup. But 
the refraction of the water bends the ray downwards, pro¬ 
ducing the same effect as though the object had been raised 
upwards, and hence it becomes visible. 

682. The transparent body through which the light passes 
is called the medium , and it is found in all cases, “ that where 
a ray of light passes obliquely from one medium into another 
of a different density , it is refracted, or turned out of its former 
course This is illustrated in the above examples, the 
water being a more dense medium than air. The refraction 
takes place at the surface of the medium, and the ray is re¬ 
fracted in its passage out of the refracting substance as well 
as into it. 



What is the color of the light which different bodies throw off? If grass 
throws off green light, what becomes of the other rays ? What, is the rate ot 
velocity with which light moves? Can we perceive any difference in the 
time which it takes an artificial light to pass to us from a great or small dis¬ 
tance? What is meant by the refraction of light? Do all transparent bodies 
refract light equally? Explain fig. 139, and show how the ray is refracted in 
passing into and out of the water. Explain fig. 140, and state the reason why 
the shilling seems to be raised up by pouring in the water. What is a medium ? 
In what direction must a ray of light pass towards the medium to be refracted ? 



OPTICS. 


187 


683. If the ra}'-, after having passed through the water, 
then strikes upon a still more dense medium, as a pane of 
glass, it will again be refracted. It is understood, that in all 
cases the raj must fall upon the refracting medium oblique¬ 
ly, in order to be refracted, for if it proceeds from one medium 
to another perpendicularly to their surfaces, it will pass 
straight through them all, and no refraction will take place. 

Thus, in fig. 141, let a. represent air, b 
water, and c is a piece of glass. The ray Fig- 14L 

<Z, striking each medium in a perpendicu¬ 
lar direction, passes through them all in a 
straight line. The oblique ray passes 
through the air in the direction of c, but 
meeting the water, is refracted in the 
dire ction of o ; then falling upon the glass, 
it i i again refracted in the direction of p, 
nearly parallel with the perpendicular 
line d. 

684. In all cases where the ray passes 
out of a rarer into a denser medium, it is 
refracted towards a perpendicular line, rais¬ 
ed from the surface of the denser medium , 
and, so , when it passes out of a denser , into 

a rarer medium , it is refracted from the same perpendicular. 

Let the medium b , fig. 142, be glass, and the medium c, 
water. The ray a, as it falls upon the medium Z>, is refracted 
towards the perpendicular line e d; 



but when it enters the water, whose 
refractive power is less than that of 
glass, it is not bent so near the per¬ 
pendicular as before, and hence it is 
refracted from, instead of towards the 
perpendicular line, and approaches 
the original direction of the ray a, g, 
when passing through the air. 

The cause of refraction appears to 
be the power of attraction, which the 
denser medium exerts on the passing 
ray; and in all cases the attracting 


Fig. 142. 



Will a ray falling perpendicularly on a medium be refracted ? Explain fig. 
141, and show how the ray e is refracted. When the ray passes out of a rarer 
into a denser medium, in what direction is it refracted ? When it passes out 
of a denser into a rarer medium, in what direction is the refraction ? Explain 
this by fig. 142. What is the cause of refraction? 


















188 


OPTICS. 


force acts in the direction of a perpendicular to the refracting 
surface. 

685. Refraction by Water .—The refraction of the rays of 
light, as they fall upon the surface of the water, is the reason 
why a straight rod, with one end in the water, and the other 
end rising above it, appears to be broken, or bent, and also to 
be shortened. 

Suppose the rod «, fig. 143, to be set with one half of its 
length below the surface of the water, and the other half 
above it. The eye being placed in an oblique direction, 
will see the lower end apparently at the point a, while the 
real termination of the rod would be at 
n; the refraction will therefore make 
the rod appear shorter by the distance 
from o to w, or one-fourth shorter than 
the part below the water really is. The 
reason why the rod appears distorted, 
or broken, is, that we judge of the di¬ 
rection of the part which is under the 
water, by that which is above it, and 
the refraction of the rays coming from 
below the surface of the water, give them a different direc¬ 
tion, when compared with those coming from that part of the 
rod which is above it. Hence, when the whole rod is below 
the water, no such distorted appearance is observed, because 
then all the rays are refracted equally. 

For the reason just explained, persons are often deceived 
in respect to the depth of water, the refraction making it ap¬ 
pear much more shallow than it really is; and there is no 
doubt but the most serious accidents have often happened to 
those who have gone into the water under such deception; 
for a pond which is really six feet deep, will appear to the 
eye only a little more than four feet deep. 

REFLECTION OF LIGHT. 

686. If a boy throws his ball a£ inst the side of a house 
swiftly, and in a perpendicular di> 3tion, it will bound back 
nearly in the line in which it wa* thrown, and he will he 
able to catch it with his hands ; but if the ball be thrown ob¬ 
liquely to the right, or left, it will bound away from the side 
of the house in the same relative direction in which it was 
thrown. 


What is the reason that a rod, with one end in the water, appears distorted 
and shorter than it really is ? Why does the water in a pond appear less deep 
nan it really is ? 






MIRRORS. 


189 



The reflection of light, so far as regards Fig. 144. 
the line of approach, and the line of leav¬ 
ing a reflecting surface, is governed by the 
same law. 

Thus, if a sun beam, fig. 144, passing 
through a small aperture in the window 
shutter < 2 , be permitted to fall upon the 
plane mirror, or looking-glass, c, d, at d 
light-angles, it will be reflected back at 
right-angles with the mirror, and therefore will pass back 
again in exactly the same direction in which it approached. 

687. But if the ray strikes the mirror Fi 145 

in an oblique direction, it will also be 
thrown off in an oblique direction, op¬ 
posite to that in which it was thrown. 

Let a ray pass towards a mirror in 
the line a, c, fig. 145, it will be reflected 
off in the direction of c, d, making the 
angles 1 and 2 exactly equal. 

The ray a, c, is called the incident ray, 
and the ray c, d, the reflected ray ; and 
it is found, in all cases, that whatever 
angle the ray of incidence makes with the 
reflecting surface, or with a perpendicular 
line drawn from the reflecting surface, ex¬ 
actly the same angle is made by the re¬ 
flected ray. 

688. From these facts, arise the general 
law in optics, that the angle of reflection is 
equal to the angle of incidence. 

The ray «, c, fig. 146, is the ray of inci¬ 
dence, and that from c to d, is the ray of 
reflection. The angles which a, c, make 
with the perpendicular line, and with the 
plane of the mirror, is exactly equal to 
those made by c, d, with the same perpendicular, and the 
same plane surface. 



MIRRORS. 

689. Mirrors are of three kinds , namely , plane , convex, and 


Suppose a sun beam fall upon a plane mirror, at right-angles with its surface, 
in what direction will it be reflected ? Suppose the ray falls obliquely on its 
surface, in what direction will it then be reflected ? What is an incident ray of 
light l What is a reflected ray of light ? What general law in optics results 
from observations on the incident and reflected rays? How many kinds of 
mirrors are there 7 









190 


MIRRORS. 


concave. They are made of polished metal, or ofglass covered 
on the hack with an amalgam of tin and quicksilver. 

Plane Mirror. —The common looking-glass is a plane 
mirror, and consists of a plate of ground glass so highly pol¬ 
ished as to permit the rays of light to pass through it with 
little interruption. On the back of this plate is placed the 
reflecting surface, which consists of a mixture of tin and 
mercury. The glass plate, therefore, only answers the pur¬ 
pose of sustaining the metallic surface on its plate,—of ad¬ 
mitting the rays of light to and from it, and of preventing its 
surface from tarnishing, by excluding the air. Could the 
metallic surface, however, be retained in its place, and not 
exposed to the air, without the glass plate, these mirrors 
would be much more perfect than they are, since, in prac¬ 
tice, glass cannot be made so perfect as to transmit all the 
rays of light which fall on its surface. 

690. When applied to the plane mirror, the angles of in¬ 
cidence and of reflection are equal, as already stated; and it 
therefore follows, that when the rays of light fall upon it 
obliquely in one direction, they are thrown off under the same 
angle in the opposite direction. 

This is the reason why the images of objects can be seen 
when the objects themselves are not visible. 

Suppose the mirror a b , fig. 147, 
to be placed on the side of a room, 
and a lamp to be set in another room, 
but so situated as that its light 
would shine upon the glass. The 
lamp itself could not be seen by the 
eye placed at e, because the parti¬ 
tion d is between them; but its im¬ 
age would be visible at e, because 
the angle of the incident ray, com¬ 
ing from the light, and that of the 
reflected ray which reaches the eye, 
are equal. 

691. An image from a plane mirror appears to he just as 
far behind the mirror, as the object is before it, so that when a 
person approaches this mirror, his image seems to come forward 
to meet him; and when he withdraws from it , his image appears 
to be moving backward at the same rate. 


Fig. 147. 



What kind of mirror is the common looking-glass ? Of what use is the glass 
plate in the construction of this mirror? Explain fig. 147, and show how the 
image of an object can be seen in a plane mirror, when the real object is 
invisible. 





MIRRORS. 


191 


If, for instance, one end of a rod, two feet long, be made 
to touch the surface of such a mirror, this end of the rod, 
and its image, will seem nearly to touch each other, there 
being only the thickness of the glass between them ; while 
the other end of the rod, and the other end of its image, will 
appear to be equally distant from the point of contact. 

The reason of this is explained on the principle that the 
angle of incidence and that of reflection is equal. 

Suppose the arrow a to Fig. 148. 

be the object reflected by 
the mirror dc , fig. 148; the 
incident rays a, flowing from 
the end of the arrow, being 
thrown back by reflection, 
will meet the eye in the 
same state of divergence 
that they would do, if they 
proceeded to the same dis¬ 
tance behind the mirror, that 
the eye is before it, as at o. 

Therefore, by the same law, the reflected rays, where they 
meet the eye at e, appear to diverge from a point A, just as 
far behind the mirror as a is before it, and consequently the 
end of the arrow most remote from the glass will appear to 
be at A, or the point where the approaching rays would meet, 
were they continued onward behind the glass. The rays 
flowing from every other part of the arrow follow the same 
law; and thus every part of the image seems to be at the 
same distance behind the mirror that the object really is before it. 

692. In a plane mirror , a person may see his whole image , 
when the mirror is only half as long as himself let him stand 
at any distance from it whatever. 

This is also explained by the law, that the angles of inci¬ 
dence and reflection are equal. If the mirror be elevated so 
that the ray of light from the eye falls perpendicularly upon 
the mirror, this ray will be thrown back by reflection in the 
same direction, so that the incident and reflected ray by 
which the image of the eyes and face are formed, will be 
nearly parallel, while the ray flowing from his feet will fall 
on the mirror obliquely, and will be reflected as obliquely in 
the contrary direction, and so of all the other rays by which 
the image of the different parts of the person is formed. 



The image of an object appears just as far behind a plane mirror, as the ob¬ 
ject is before it; explain fig. 148, and show why this is the case. What must 
be the comparative length of a plane mirror in which a person may see his 
whole image ? 




92 


MIRRORS. 


Thus, suppose the 
mirror c e , fig. 149, to 
be just half as long as 
the arrow placed be¬ 
fore it, and suppose the 
eye to be placed at a. 

Then the ray a e , pro¬ 
ceeding from the eye at 
«, and falling perpendic¬ 
ularly on the glass at 
c, will be reflected back 
to the eye in the same 
line, and this part of the image will appear at b , in the same 
line, and at the same distance behind the glass, that the ar¬ 
row is before it. But the ray flowing from the lower extrem¬ 
ity of the arrow, will fall on the mirror obliquely as at e, and 
will be reflected under the same angle to the eye, and there¬ 
fore the extremity of the image-, appearing in the direction of 
the reflected ray, will be seen at d. The rays flowing from 
the other parts of the arrow, will observe the same law, and 
thus the whole image is seen distinctly, and in the same po¬ 
sition as the object. 

To render this still more obvious, suppose the mirror to be 
removed, and another arrow to be placed in the position 
where its image appears, behind the mirror, of the same 
length as the one before it. Then the eye, being in the same 
position as represented in the figure, would see the different 
parts of the real arrow in the same direction that it before 
saw the image. Thus, the ray flowing from the upper ex¬ 
tremity of the arrow, would meet the eye in the direction of 
b c, while the ray, coming from the lower extremity, would 
fall on it in the direction of e d. 

693. Convex Mirror. —A 
convex mirror is a part of a 
sphere , or globe , refecting from 
the outside. 

Suppose fig. 150 to be a 
sphere, then the part from a to 
o, would be a section of the 
sphere. Any part of a hollow 
ball of glass, with an amal¬ 
gam of tin and quicksilver 
spread on the inside, or any 
part of a metallic globe pol¬ 
ished on the outside, would form a convex mirror. 


Fig. 150. 



Fig. 149. 









MIRRORS. 


193 


The axis of a convex mirror, is a line as c b, passing 
through its centre. 

694. Divergent and Convergent 
Rays. —Rajs of light are said to 
diverge , when they proceed from 
the same point, and constantly 
recede from each other, as from 
the point a, fig. 151. Rays of 
light are said to converge, when 
they approach each other in such 
a direction as finally to meet at a point, as at £, fig. 151. 

The image formed by a plane mirror, as we have already 
seen, is of the same size as the object, but the image reflected 
from the convex mirror is always smaller than the object. 

The law which governs the passage of light with respect 
to the angles of incidence and reflection, to and from the 
convex mirror, is the same as already stated, for the plane 
mirror. 

695. From the surface of a plane mirror, parallel rays 
are reflected parallel; but the convex mirror causes parallel 
rays falling on its surface to diverge , by reflection. 

To make this understood, 
let 1, 2, 3, fig. 152, be paral¬ 
lel rays, falling on the sur¬ 
face of the convex reflector, 
of which a would be the cen¬ 
tre, were the reflector a 
whole sphere. The ray 2 is 
perpendicular to the surface 
of the mirror, for when con¬ 
tinued in the same direction, 
it strikes the axis, or centre 
of the circle a. The two 
rays, 1 and 3, being parallel 
to this, all three would fall 
on a plane mirror in a perpendicular direction, and conse¬ 
quently would be reflected in the lines of their incidence. 

In what part of the image, fig. 149, are the incidental and reflected rays 
nearly parallel ? Why does the image of the lower part of the arrow appear 
at d ? Suppose the mirror, fig. 149, to be removed, and an arrow of the same 
length to be placed where the image appeared, would the direction of the rays 
from the arrow be the same that they were from the image ? What is a convex 
mirror? What is the axis of a convex mirror? What are diverging rays? 
What are converging rays ? What law governs the passage of light from and 
to the convex mirror ? Are parallel rays falling on a convex mirror, reflected 
parallel ? Explain fig. 152. 


Fig. 152. 

0 









194 


MIRRORS. 


But the obliquity of the convex surface, it is obvious, will 
render the direction of the rays 1 and 3, oblique to that sur¬ 
face, for the same reason that 2 is perpendicular to that part 
of the circle on which it falls. Rays falling on any part 
of this mirror, in a direction which, if continued through the 
circumference, would strike the centre, are perpendicular to 
the side where they fall. Thus, the dotted lines, c a and d a, 
are perpendicular to the surface, as well as 2. 

Now the reflection of the ray 2, will be back in the line 
of its incidence, but the rays 1 and 3, falling obliquely, are 
reflected under the same angles at which they fall, and there¬ 
fore their lines of reflection will be as far without the per¬ 
pendicular lines c a, and d a , as the lines of their incident 
rays, 1 and 3, are within them, and consequently they will 
diverge in the direction of e and o; and since we always see 
the image in the direction of the reflected ray, an object 
placed at 1, would appear behind the surface of the mirror at 
«, or in the direction of the line o n. 

696. Plane Surfaces .—Perhaps the subject of the convex 
mirror will be better understood, by considering its surface to 
be formed of a number of plane surfaces, indefinitely small. 
In this case, each point from which a ray is reflected, would 
act in the same manner as a plane mirror, and the whole, in 
the manner of a number of minute mirrors inclined from 
each other. 

Suppose a and &, fig. 153, 
to be the points on a convex 
mirror, from which the two 
parallel rays, c and d, are re¬ 
flected. Now, from the sur¬ 
face of a plane mirror, the 
reflected rays would be paral¬ 
lel, whenever the incident 
ones are so, because each 
will fall upon the surface 
under the same angles. But 
it is obvious in the present 
case, that these rays fall upon the surfac es, a and 5, under 
different angles, as respects the surfaces, c approaching in a 
more oblique direction than d; consequently c is reflected 
more obliquely than d , and the two reflected rays, instead of 
being parallel as before, diverge in the direction of n and o. 

mirrora ? S th ® aCti ° n ° f the convex mirror illustrated by a number of plan* 






MIRRORS. 


195 


697. Again, the two con¬ 
verging- rays a and b, fig. 

154, without the interposi¬ 
tion of the reflecting sur¬ 
faces, would meet at c, but 
because the angles of re¬ 
flection are equal to those 
of incidence, and because 
the surfaces of reflection 
are inclined from each other, 
these rays are reflected less 
convergent, and instead of 
meeting at the same dis¬ 
tance before the mirror that c is behind it, are sent off in the 
direction of e, at which point they meet. 

698. “ Thus parallel rays falling on a convex mirror, are 
rendered divergent by reJU ction; converging rays are made 
less convergent, or parallel, and diverging rays more divergent .” 

The effect of the convex mirror, therefore, is to disperse 
the rays of light in all directions; and it is proper here to 
remind the pupil, that although the rays of light are repre¬ 
sented on paper by single lines, there are in fact probably 
millions of rays, proceeding from every point of all visible 
bodies. Only a comparatively small number of these rays, 
it is true, can enter the eye, for it is only by those which 
proceed in straight lines from the different parts of the ob¬ 
ject, and enter the pupil, that the sense of vision is excited. 

Now, to conceive how exceedingly small must be the pro¬ 
portion of light thrown off, from any visible object which 
enters the eye, we must consider that the same object re¬ 
flects rays in every other direction, as well as in that in which 
it is seen. Thus, the gilded ball on the steeple of a church 
may be seen by millions of persons at the same time, who 
stand upon the ground ; and were millions more raised above 
these, it would.be visible to all. 

When, therefore, it is said, that the convex mirror dispers¬ 
es the rays of light which fall upon it from any object, and 
when the direction of these reflected rays are shown only by 
single lines, it must be remembered, that each line represents 
pencils of rays, and that the light not only flows from the 


Fig. 154. 



Explain fig. 154. What effect does the convex mirror have upon parallel 
rays by reflection? What is its effect on converging rays? What is its 
effect on diverging rays ? Do the rays of light proceed only from the extremi¬ 
ties of objects, as represented in figures, or from all their parts ? Do all the 
rays of light proceeding from an object enter the eye, or only a few of them ? 



MIRRORS 


196 

parts of the object thus designated, but from all the other 
parts. Were this not the case, the object would be visible 
only at certain points. 

699. Curved Images.—The images of objects reflected from 
the convex mirroi , appear curved , because their different parts 
are not equally distant from its surface. 

If tne object a be placed 
obliquely before the con¬ 
vex mirror, fig. 155, then 
the converging rays from 
its two extremities falling 
obliquely on its surface, 
would, were they prolong¬ 
ed through the mirror, 
meet at the point c, behind 
it. But instead of being 
thus continued, they are thrown back by the mirror in less 
convergent lines, which meet the eye at <?, it being, as we 
have seen, one of the properties of this mirror, to reflect con¬ 
verging rays less convergent than before. 

The image being always seen in the direction from which 
the rays approach the eye, it appears behind the mirror at d. 
If the eye be kept in the same position, and the object, a, be 
moved further from the mirror, its image will appear smaller, 
in a proportion inversely to the distance to which it is re¬ 
moved. Consequently, by the same law, the two ends of a 
straight object will appear smaller than its middle, because 
they are further from the reflecting surface of the mirror. 
Thus, the images of straight objects, held before a convex 
mirror, appear curved, and for the same reason, the features 
of the face appear out of proportion, the nose being too large, 
and the cheeks too small, or narrow. 

700. Why Objects Appear Large or Small .—Objects ap¬ 
pear to us large or small, in proportion to the angle which 
the rays of light, proceeding from their extreme parts, form, 
when they meet at the eye. For it is plain that the half of 
any object will appear under a less angle than the whole, 
and the quarter under a less angle still. Therefore the 
smaller an object is, the smaller will be the angle under 

What would be the consequence, if the rays of light proceeded only from 
the parts of an object shown in diagrams ? Why do the images of objects 
reflected from convex mirrors appear curved ? Why do the features of the 
face appear out of proportion, by this mirror? Why does an image reflected 
from a convex surface appear smaller than the object? Why does the half of 
an object appear to the eye smaller than the whole? 




MIRRORS. 


197 


which it will appear at a given distance. If, then, a mirror 
makes the angle under which an object is seen, smaller, the 
object itself will seem smaller than it really is. Hence the 
image of an object, when reflected from the convex mirror , ap¬ 
pears smaller than the object itself This will be understood 
by fig. 156. 

Suppose the rays flow¬ 
ing from the extremities 
of the object a , to be re¬ 
flected back to c, under 
the same degrees of con¬ 
vergence at which they 
strike the mirror; then, 
as in the plane mirror, 
the image d , would ap¬ 
pear of the same size 
as the object a; for if 
the rays from a were 
prolonged behind the mirror, they would meet at b , but form¬ 
ing the same angle, by reflection, that they would do, if thus 
prolonged, the object seen from b , and its image from c, would 
appear of the same dimensions. 

But instead of this, the rays from the arrow a, being ren¬ 
dered less convergent by reflection, are continued onward, 
and meet the eye under a more acute angle than at c, the 
angle under which they actually meet, being represented at 
e , consequently the image of the object is shortened in pro¬ 
portion to the acuteness of this angle, and the object appears 
diminished as represented at o. 

701. The image of an object appears less , as the object is 
removed to a greater distance from the mirror. 

To explain this, let us sup- Fi s- im¬ 

pose that the arrow a , fig. 157, 
is diminished by reflection 
from the convex surface, so 
that its image appearing at d , 
with the eye at c, shall seem 
as much smaller in proportion 
to the object, as <7is less than 
a. Now, keeping the eye at 
the same distance from the 
mirror, withdraw the object, so that it shall be equally dis- 

Suppose the angles c and b, fig. 156, are equal, will there be any difference 
between the size of the object and its image? How is the image affected 
when the object is withdrawn from the surface of a convex mirror? 

17* 



Fig. 136. 





MIRRORS. 


198 

tant with the eye, and the image will gradually diminish, as 
the arrow is removed. 

702. The reason will be 
made plain by the next fig¬ 
ure ; for as the arrow is moved 
backwards, the angle at c, fig. 

158, must be diminished, be¬ 
cause the rays flowing from 
the extremities of the object fall 
a greater distance before they 
reach the surface of the mir¬ 
ror ; and as the angles of the 
reflected rays bear a proportion to those of the incident ones, 
so the angle of vision will become less in proportion, as the 
object is withdrawn. The effect, therefore, of withdrawing 
the object, is first to lessen the distance between the converg¬ 
ing rays, flowing from it, at the point where they strike the 
mirror, and as a consequence, to diminish the angle under 
which the reflected rays convey its image to the eye. 

703. Why the Image seems near the surface .—In the plane 
mirror, as already shown, the image appears exactly as far 
behind the mirror as the object is before it, but the convex 
mirror shows the image just under the surface, or, when the 
object is removed to a distance, a little way behind it. To 
understand the reason of this difference, it must be remem¬ 
bered, that the plane mirror makes the image seem as far 
behind, as the object is before it, because the rays are re¬ 
flected in the same relative position at which they fall upon 
its surface. Thus parallel rays are reflected parallel; diver¬ 
gent rays equally divergent, and convergent rays equally 
convergent. But the convex mirror, as also above shown, 
(698) reflects convergent rays less convergent, and divergent 
rays more divergent, and it is from this property of the con¬ 
vex mirror that the image appears near its surface, and not 
as far behind it as the object is before it, as in the plane mirror. 

Let us suppose that a, fig. 159, 
is a luminous point, from which a 
pencil of diverging rays fall upon 
a convex mirror. These rays, as 
already demonstrated, will be re¬ 
flected more divergent, and conse¬ 
quently will meet the eye at e, in 
a wider state of dispersion than 
they fell upon the mirror at o. Now, 
as the image will appear at the 
point where the diverging raye 


Fig. 159. 



Fig. 158. 



MIRRORS. 


19*9 


would converge to a focus in a contrary direction, were they 
prolonged behind the mirror, so it cannot appear as far be¬ 
hind the reflecting surface as the object is before it, for the 
more widely the rays meeting at the eye are separated, the 
shorter will be the distance at which they will come to a 
point. The image will, therefore, appear at », instead of 
appearing at an equal distance behind the mirror that the 
object a is before it. 

704. Concave Mirror. —The reflection of the concave mir¬ 
ror takes place from its inside , or concave surface , while that 
of the convex mirror is from the outside , or convex surface. 
Thus the section of a metallic sphere, polished on both sides, 
is both a concave and convex mirror, as one or the other side 
is employed for reflection. 

The effect and phenomena of this mirror will therefore be, 
in many respects, directly the contrary from those already 
detailed in reference to the convex mirror. 

From the plane mirror, the relation of the incident rays are 
not changed by reflection; from the convex mirror they are dis¬ 
persed ; but the concave mirror renders the rays reflected from 
it more convergent , and tends to concentrate them into a focus. 

The surface of the concave mirror, like that of the convex, 
may be considered as a great number of minute plane mirrors, 
inclined to each other at certain angles, in proportion to its 
concavity. 

705. The laws of incidence and reflection afe the same, 
when applied to the concave mirror, as those already ex¬ 
plained in reference to the other mirrors. 

Plane Mirrors Inclined. —In refer¬ 
ence to the concave mirror, let us, in 
the first place, examine the effect of 
two plane mirrors inclined to each 
other, as in fig. 160, on parallel rays 
of light. The incident rays, a and 
b , being parallel before they reach the 
reflectors, are thrown off at unequal 
angles in respect to each other, for b 
falls on the mirror more obliquely than 
a, and consequently is thrown off 

Explain figures 157 and 158, and show the reason why the images are di¬ 
minished when the objects are removed from the convex mirror. What is said 
to be the first effect of withdrawing the object from a concave surface, and 
-what the consequence on the angle of reflected rays ? Explain the reason why 
the image appears near the surface of the convex mirror. What is the shape 
of the concave mirror, and in what respect does it differ from the convex mir 
ror 1 How may convex and concave mirrors be united in the same instrument f 


Fig. UK). 





200 


MIRRORS. 


more obliquely in a contrary direction, therefore, the angles of 
reflection being equal to those of incidence, the two rays 
meet at c. Thus we see that the effect of two plane mirrors 
inclined to each other , is to make parallel rays converge and 
meet in a focus. 

The same result would take place, whether the mirror 
was one continued circle, or an infinite number of small mir¬ 
rors inclined to each other in the same relation as the differ¬ 
ent parts of the circle. 

The effect of this mirror, as we have seen, being to render 
parallel rays convergent, the same principle will render di¬ 
verging rays parallel, and converging rays still more conver¬ 
gent. 

706. Focus of a Concave Mirror. —The focus of a concave 
mirror is the point where the rays are brought together by 
reflection. The centre of concavity in a concave mirror, is the 
centre of the sphere, of which the mirror is a part. In all 
concave mirrors, the focus of parallel rays, or rays falling 
directly from the sun, is at the distance of half the semi¬ 
diameter of the sphere, or globe, of which the reflector is a 
part. 

Thus, the paral- Fig* 161. 

lei rays 1, 2, 3, &c., 
fig. 161, all meet at 
the point o, which 
is half the distance 
between the centre 
a, of the whole 
sphere, and the sur¬ 
face of the reflector, 
and therefore one 
quarter the diame¬ 
ter of the whole 
sphere, of which the 
mirror is a part. 

707. Principal 
Focus. —In concave 
mirrors, of all dimensions, the reflected rays follow the same 
law; that is, parallel rays meet and cross each other at the 



What is the difference of effect between the concave, convex and nlane 
a , 6 reflec , ted W ? In what respect may the concave mirror be 
mTmf’ At wtat n S er Plane mirrors ? What is the focus of a concave 
? m f distance from its surface is the focus of parallel rays in this 
murroi ? What is the principal focus of a concave mirror ’ 














MIRRORS. 


201 

distance of one fourth the diameter of the sphere of which 
they are sections. This point is called the principal focus of 
the reflector. 

But if the incident rays are divergent, the focus will be 
removed to a greater distance from the surface of the mirror, 
than when they are parallel, in proportion to their diver¬ 
gency. 

This might be inferred from the 
general laws of incidence and re¬ 
flection, but will be made obvious 
by fig. 162, where the diverging 
rays 1, 2, 3, 4, form a focus at the 
point o, whereas, had they been 
parallel, their focus would have been 
at a. That is, the actual focus 
is at the centre of the sphere, 
instead of being half way between 
the centre and circumference, as is 
the case when the incident rays are 
parallel. The real focus, therefore, is beyond, or without, the 
principal focus of the mirror. 

708. By the same law, converging rays will form a point 
within the principal focus of the mirror. 

Thus, were the rays falling on 
the mirror, fig. 163, parallel, the 
focus would be at a; but in con¬ 
sequence of their previous conver- 
gency, they are brought together 
at a less distance than the princi¬ 
pal focus, and meet at o. 

The concave mirror , when the 
object is nearer to it than the prin¬ 
cipal focus, presents the image 
larger than the object, erect, and 
behind the mirror. 

To explain this, let us suppose the object a, fig. 164, to 
be placed before the mirror, and nearer to it than the prin¬ 
cipal focus. Then the rays proceeding from the extremities 
of the object without interruption, would continue to diverge 
in the lines o and n , as seen behind the mirror; but, by re¬ 
flection, they are made to diverge less than before, and con* 



Fig. 162. 



If the incident rays are divergent, where will be the focus ? If the incident 
rays are convergent, where will be the focus ? When will the image from a 
concave mirror be larger than the object, erect, and behind the mirror 1 





MIRRORS. 


202 

sequently to make 
the angle under 
which they meet 
more obtuse at the 
eye b , than it would 
be if they continu* 
ed onward to e, 
where they would 
have met without 
reflection. The re¬ 
sult, therefore, is to 
render the image A, 
upon the eye, as 
much larger than 
the object a , as the 
angle at the eye is 
more obtuse than the angle at e. 

709. On the contrary, if the object is placed more remote 
from the mirror than the principal focus, and between the 
focus and the centre of the sphere of which the reflector is 
a part, then the image will appear inverted on the contrary 
side of the centre, and farther from the mirror than the ob¬ 
ject ; thus, if a lamp be placed obliquely before a concave 
mirror, as in 

fig. 165, its Fig- 165. 

image will be 
seen inverted 
in the air, on 
the contrary 
side of a per¬ 
pendicularline 
through the 
centre of the 
mirror. 

710. Curious Deceptions by Concave Mirrors .—From the 
property of the concave mirror to form an inverted image of 
the object suspended in the air, many curious and surprising 
deceptions may be produced. Thus, when the mirror, 
the object, and the light, are placed so that they cannot 
be seen, (which may be done by placing a screen before 


Explain fig. 164, and show why the image is larger than the object. When 
will the image from the concave mirror be inverted, and before the mirror ? 
What property has the concave mirror, by which singular deceptions may be 
produced i What are these deceptions ? 



Fig. 164. 





MIRRORS. 


203 

the light, and permitting the reflected rays to pass through 
a small aperture in another screen,) the person mistakes the 
image of the object for its reality, and not understanding the 
deception, thinks he sees persons walking with their heads 
downwards, and cups of water turned bottom upwards, with¬ 
out spilling a drop. Again, he sees clusters of delicious 
fruit, and when invited to help himself, on reaching out his 
hand for that purpose, he finds that the object either suddenly 
vanishes from his sight, owing to his having moved his eye 
out of the proper range, or that it is intangible. 

This kind of deception may be illustrated by any one who 
has a concave mirror only of three or four inches in diameter, 
in the following manner : 

Suppose the tumbler a, to be filled with water, and placed 
beyond the principal focus of the concave mirror, fig. 166, 
and so managed as to be hid from the eye c, by the screen, 
b. The lamp by which the tumbler is illuminated must also 
be placed behind the screen, and near the tumbler. To a 
person placed at c, the tumbler with its contents will appear 
inverted at e, and suspended in the air. By carefully moving 
forward, and still keeping the eye in the same line with re¬ 
spect to the mirror, the person may pass his hand through 
the shadow of the tumbler ; but without such conviction, any 
one unacquainted with such things, could hardly be made to 
believe that the image was not a reality. 

Fig. 166. 



By placing another screen between the mirror and the 
image, and permitting the converging rays to pass through 


Describe the manner in which a tumbler with its contents may be made to 
seem inverted in the air. 









204 


MIRRORS. 


an aperture in it, the mirror may be nearly covered from the 
eye, and thus the deception would be increased. 

711. Amusing Effects of the Concave Mirror. —The image 
reflected from a concave mirror, moves in the same direction 
with the object, when the object is within the principal focus ; 
but when the object is more remote than the principal focus, 
the image moves in a contrary direction from the object, be¬ 
cause the rays then cross each other. If a man place him¬ 
self directly before a large concave mirror, but farther from it 
than the centre of concavity, he will see an inverted image 
of himself in the air, between him and the mirror, but less 
than himself. And if he hold out his hand towards the mir¬ 
ror, the hand of his image will come out toward his hand, 
and he may imagine that he can shake hands with his im¬ 
age. But if he reach his hand further towards the mirror, 
the hand of the image will pass by his hand, and come be¬ 
tween his hand and his body; and if he move his hand to¬ 
ward either side, the hand of the image will move in a con¬ 
trary direction, so that if the object moves one way, the 
image will move the other. 

712. Heat Produced by this Mirror. —The concave mirror 
having the property of converging the rays of light, is equal¬ 
ly efficient in concentrating the rays of heat, either separately, 
or with the light. When, therefore, such a mirror is presented 
to the rays of the sun, it brings them to a focus, so as to 
produce degrees of heat in proportion to the extent and per¬ 
fection of its reflecting surface. A metallic mirror of this 
kind, of only four or six inches in diameter, will fuse metals, 
set wood on fire, &c. 

718. Experiment with a Hot Ball. —As the parallel rays of 
heat or light are brought to a focus at the distance, of one 
quarter of the diameter of the sphere, of which the reflector 
is a section, so if a luminous or heated body be placed at 
this point, the rays from such body passing to the mirror will 
be reflected, from all parts of its surface, in parallel lines ; and 
the rays so reflected by the same law, will be brought to a 
focus by another mirror standing opposite to this. 

Suppose a red hot ball to be placed in the principal focus 
of the mirror a, fig. 167, the rays of heat and light proceed¬ 
ing from it will be reflected in the parallel lines 1 , 2, 3 , &c. 

Why does the image move in a contrary direction from its object, when the 
object is beyond the principal focus ? Will the concave mirror concentrate 
the rays of heat, as well as those of light ? Suppose a luminous body be 
placed in the focus of a concave mirror, in whaftlirection will its rays be re 
fleeted ? 




MIRRORS. 


205 



The reason of this is the same as that which causes parallel 
rays, when falling on the mirror, to be converged to a focus. 
The angles of incidence being equal to those of reflection, it 
makes no difference in this respect, whether the rays pass to 
or from the focus. In one case, parallel incident rays from 
the sun, are concentrated by reflection; and in the other, 
incident diverging rays, from the heated ball, are made 
parallel by reflection. 

The rays, therefore, flowing from the hot ball to the mirror 
a, are thrown into parallel lines by reflection, and these re¬ 
flected rays, in respect to the mirror 6, become the rays of 
incidence, which are again brought to a focus by reflection. 

Thus the heat of the ball, by being placed in the focus of 
one mirror, is brought to a focus by the reflection of the other 
mirror. 

Several striking experiments may be made with a pair of 
concave mirrors placed facing each other, as in the figure. 
If a red hot ball be placed in the focus of «, and some gun¬ 
powder in the focus of b , the mirrors being ten or twenty feet 
apart, according to their dimensions, the powder will flash 
by the heat of the ball, concentrated by the second mirror. 
To show that it is not the direct heat of the ball which sets 
f)re to the powder, a paper screen may be placed between 
the mirrors until every thing is ready. The operator will 
then only have to remove the screen in order to flash the 
powder. 

To show that heat and light are separate principles, place 


Explain fig. 167, and show why the rays from the focus of a are concentra¬ 
ted in the focus b. What curious experiments may be made by two concave 
mirrors placed opposite to each other ? 

18 


























206 


LENSES. 


a piece of phosphorus in the focus of 5, and when the ball is 
so cool as not to be luminous, remove the screen, and the 
phosphorus will instantly inflame. 

REFRACTION BY LENSES. 

714. A Lens is a transparent body, generally made of glass , 
and so shaped that the rays of light in passing through it are 
either collected together or dispersed. Lens is a Latin word, 
which comes from lentile , a small flat bean. 

It has already been shown, that when the rays of light 
pass from a rarer to a denser medium, they are refracted, or 
bent out of their former course, except when they happen to 
fall perpendicularly on the surface of the medium. (651.) 

The point where no refraction is produced on perpendicu¬ 
lar rays, is called the axis of the lens, which is a right line 
passing through its centre, and perpendicular to both its 
surfaces. 

In every beam of light the middle ray is called its axis. 

Rays of light are said to fall directly upon a lens, when 
their axes coincide with the axes of the lens; otherwise they 
are said to fall obliquely. 

The point at which the rays of the sun are collected, by 
passing through a lens, is called the principal focus of that 
lens. 

715. Lenses are of various kinds, and have received cer¬ 
tain names, depending on their shapes. The different kinds 
are shown at fig. 168. 

Fig. 168. 


i 



? 


A prism , seen at a, has two plane surfaces, a r , and a s , 
inclined to each other. 

A plane glass , shown at b , has two plane surfaces, paral¬ 
lel to each other. 



How may it be shown that heat and light are distinct principles ? What is 
a lens ? What is the axis of a lens ? In what part of a lens is no refraction 
produced ? Where is the axis of a beam of light ? When are rays of light said 
to fall directly upon a lens ? 









LENSES. 


207 


A spherical lens , c, is a ball of glass, and has every part 
of its surface at an equal distance from the centre. 

A double-convex lens, e?, is botinded by two convex sur¬ 
faces, opposite to each other. 

A plano-convex lens , e, is bounded by a convex surface on 
one side, and a plane on the other. 

A double-concave lens, f is bounded by two concave spher¬ 
ical surfaces, opposite to each other. 

A plano-concave lens , g , is bounded by a plane surface on 
one side and a concave one on the other. 

A meniscus, A, is bounded by one concave, and one convex 
spherical surface, which two surfaces meet at the edge of 
the lens. 

A concavo-convex lens, i, is bounded by a concave, and con¬ 
vex surface, but which diverge from each other, if continued. 

The effects of the prism on the rays of light will be shown 
in another place. The refraction of the plane glass bends 
the parallel rays of light equally towards the perpendicular, 
as already shown. The sphere is not often employed as a 
lens, since it is inconvenient in use. 

716. Convex Lens. —The effect of the convex lens, by in¬ 
creasing the visual angle, is to magnify all objects seen through it. 

717. Focal Distance. —The focal distances of convex 
lenses, depend on their degrees of convexity. The focal dis¬ 
tance of a single , or plano-convex lens, is the diameter of a 
sphere, of which it is a section. 

If the whole circle, Fi s- l 69 - 

fig. 169, be consider¬ 
ed the circumference 
of a sphere, of which 
the plano-convex lens 
b, a , is a section, then 
the focus of parallel 
rays, or the principal 
focus, will be at the 
opposite side of the 
sphere, or at c. 

718. The focal dis¬ 
tance of a double convex lens, is the radius, or half the diame¬ 
ter of the sphere of which it is a part. Hence the plano-con- 



How many kinds of lenses are mentioned ? What is the name of each ? 
How are each of these lenses bounded? What is the effect of the convex lens? 
On what do the focal distances of convex lenses depend ? What is the focal 
distance of any plano-convex lens ? 









208 


LENSES. 


vex lens, being one half of the double convex lens, the latter 
has about twice the refractive power of the former; for the 
rays suffer the same degree of refraction in passing out of the 
one convex surface, that they do in passing into the other. 

The shape of the 
double convex lens, d 
e, fig. 170, is that of two 
plano-convex lenses, 
placed with their 
plane surfaces in con¬ 
tact, and consequent¬ 
ly the focal distance 
of this lens is near¬ 
ly the centre of the 
sphere of which one 
of its surfaces is a 
part. If parallel rays 
fall on a convex lens, it is evident that the ray only, which 
penetrates the axis and passes towards the centre of the 
sphere, will proceed without refraction, as shown in the above 
figures. All the others will be refracted so as to meet the 
perpendicular ray at a greater or less distance, depending on 
the convexity of the lens. 

719. If diverging rays fall on the surface of the same lens, 
they will, by refraction, be rendered less divergent, parallel 
or convergent, according to the degrees of their divergency, 
and the convexity of the surface of the lens. 

Thus, the di¬ 
verging rays 1, Fig. 171. 

2, &c., fig. 171, 
are refracted by 

the lens a o, in - 

adegreejustsuf-_ 

ficient to render _ 

them parallel, 
and therefore, ‘ 

would pass off - 

in right lines, 
indefinitely, or 
without ever forming a focus. 

720. It is obvious by the same law, that were the ravs 


What is the focal distance of the double convex lens T What is the shape of 
the double convex lens ? How are divergent rays affected by passing through 
a convex lens ? 


















LENSES. 


209 


less divergent, or were the surface of the lens more convex, 
the rays in fig. 171 would become convergent, instead 0 !" 
parallel, because the same refractive power which would 
render divergent rays parallel, would make parallel rays 
convergent, and converging rays still more convergent. 

Thus the pencils of converging rays, 
fig. 172, are rendered still more con¬ 
vergent by their passage through the 
lens, and are therefore brought to a 
focus nearer the lens, in proportion to 
their previous convergency. 

721. The eye glasses of spectacles 
for old people are double convex lens¬ 
es, more or less spherical, according to 
the age of the person, or the magni¬ 
fying power required. 

Burning Glass. —The common burning glasses, which 
are used fox lighting cigars, and sometimes for kindling fires, 
are also convex lenses. Their effect is to concentrate to a 
focus, or point, the heat of the sun which falls on their whole 
surface; and hence the intensity of their effects is in propor¬ 
tion to the extent of their surfaces, and their focal lengths. 

One of the largest burning glasses ever constructed, was 
made by Mr. Parker, of London. It was three feet in diam¬ 
eter, with a focal distance of three feet nine inches. But 
in order to increase its power still more, he employed an¬ 
other lens about a foot in diameter, to bring its rays to a 
smaller focal point. This apparatus gave a most intense de¬ 
gree of heat, when the sun was clear, so that 20 grains r,i 
gold were melted by it in 4 seconds, and ten grains of pla- 
tina, the most infusible of all metals, in 3 seconds. 

722. It has been explained, that the reason why the con¬ 
vex mirror diminishes the images of objects is, that the rays 
which come to the eye from the extreme parts of the object 
are rendered less convergent by reflection, from the convex 
surface, and that, in consequence, the angle of vision is made 
more acute. 

Now, the refractive power of the convex lens has exactly 
the contrary effect, since by converging the rays flowing 
from the extremities of an object, the visual angle is ren- 



What is its effect on parallel rays ? What is its effect on converging rays T 
What kind of lenses are spectacle glasses for old people ? What is said to be 
the diameter of Mr. Parker’s great convex lens ? What is the focal distance 
of this lens ? What is said of its heating power? 

18* 




LENSES 


210 

dered more obtuse, and therefore all objects seen through it 
appear magnified. 

Suppose the object a, 
fig. 173, appears to the 
naked eye of the length 
represented in the draw¬ 
ing. Now, as the rays 
coming from each end of 

the object, form by their convergence at the eye, the visual 
angle , or the angle under which the object is seen, and we 
call objects large or small in proportion as this angle is ob¬ 
tuse or acute, if therefore the object a be withdrawn further 
from the eye, it is apparent that the rays o , o , proceeding 
from its extremities, will enter the eye under a more acute 
angle, and therefore, that the object will appear diminished in 
proportion. This is the reason why things at a distance ap¬ 
pear smaller than when near us. When near, the visual 
angle is increased, and when at a distance it is diminished. 

723. The effect of the convex lens 

is to increase the visual angle, by Fig. 174. 

bending the rays of light coming 
from the object, so as to make them 
meet at the eye more obtusely; and 
hence it has the same effect, in re¬ 
spect to the visual angle, as bringing 
the object nearer the eye. This is 
shown by fig. 174, where it is obvi¬ 
ous, that did the rays flowing from 
the extremities of the arrow meet the 
eye without refraction, the visual an¬ 
gle would be less, and therefore the object would appear 
shorter. Another effect of the convex lens, is to enable us to 
see objects nearer the eye, than without it, as will be ex¬ 
plained under the article vision. 

Now, as the rays of light flow from all parts of a visible • 
object of whatever shape, so the breadth, as well as the 
length, is increased by the convex lens, and thus the whole 
object appears magnified. 

724. Concave Lens. — The effect of the concave lens is di¬ 
rectly opposite to that of the convex. In other terms, by a 

What is the visual angle ? Why does the same object, when at a distance, 
appear smaller than when near ? What is the effect of the convex lens on the 
visual angle ? Why does an object appear larger through the convex lens than 
otherwise ? What is the effect of the concave lens ? What effect does this 
Jens have upon parallel, diverging, and converging rays ? 



Fig. 173. 








VISION. 


211 

concave lens, parallel rays are rendered diverging, converg¬ 
ing rays have their convergency diminished, and diverging 
rays have their divergency increased, according to the con 
cavity of the lens. 

These glasses, therefore, exhibit things smaller than they 
really are, for by diminishing the convergence of the rays 
coming from the extreme points of an object, the visual an¬ 
gle is rendered more acute, and hence the object appears di¬ 
minished by this lens, for the opposite reason that it is in¬ 
creased by the convex lens. This will be made plain by the 
two following diagrams. 

Suppose the object a 6, fig. 

175, to be placed at such a 
distance from the eye, as to 
give the rays flowing from it, 
the degrees of convergence 
represented in the figure, and 
suppose that the rays enter 
the eye under such an angle 
as to make the object appear 
two feet in length. 

Now, the length of the 
same object, seen through the 
concave lens, fig. 176, will 
appear diminished, because 
the rays coming from it are 
bent outwards, or made less 
convergent by refraction, as 
seen in the figure, and conse¬ 
quently the visual angle is 
more acute than when the same object is seen by the naked 
eye. Its length, therefore, will appear less, in proportion as 
the rays are rendered less convergent. 

The spectacle glasses of short-sighted people are concave 
lenses, by which the images of objects are formed further 
back in the eye than otherwise, as will be explained under 
the next article. 

VISION. 

725. In the application of the principles of optics to the 
explanation of natural phenomena, it is necessary to give a 

Why do objects appear smaller through this glass than they do to the naked 
eye ? Explain figures 175 and 176, and show the reason why the same object 
appears smaller through 176. What defect in the eye requires concave lenses 7 
What is the most perfect of all optical instruments 7 


Fig. 175. 








2)2 


VISION. 


description of the most perfect of all optical instruments, the 
cye 


Fig. 177. 


726. Fig. 177 is a 



vertical section of 
the human eye. Its 
form is nearly globu¬ 
lar, with a slight pro¬ 
jection or elongation 
in front. It consists 
of four coats, or 
membranes; name¬ 
ly, the sclerotic , the 


cornea , the choroid , N ^^===== == =s=s^^^ 

and the retina. It 
has two fluids con¬ 
fined within these membranes, called the aqueous , and the 
vitreous humors, and one lens, called the crystalline. The 
sclerotic coat is the outer and strongest membrane, and its 
anterior part is well known as the white of the eye. This 
coat is marked in the figure a, a, a, a. It is joined to the 
cornea b, b , which is the transparent membrane in front of 
the eye, through which we see. The choroid coat is a thin, 
delicate membrane, which lines the sclerotic coat on the 
inside. On the inside of this lies the retina , d , d, d , d, which 
is the innermost coat of all, and is an expansion, or continu¬ 
ation, of the optic nerve o. This expansion of the optic nerve 
is the immediate seat of vision. The iris, o, o, is seen through 
the cornea, and is a thin membrane, or curtain of different 
colors in different persons, and therefore gives color to the 
eyes. In black eyed persons it is black, in blue eyed per¬ 
sons it is blue, &c. Through the iris, is a circular opening, 
called the pupil, which expands or enlarges when the light is 
faint, and contracts when it is too strong. The space 
between the iris and the cornea is called the anterior chamber 
of the eye, and is filled with the aqueous humor, so called 
from its resemblance to water. Behind the pupil and iris is 
situated the crystalline lens e , which is a firm and perfectly 
transparent body, through which the rays of light pass from 
the pupil to the retina. Behind the lens is situated the 


What is the form of the human eye ? How many coats, or membranes, has 
the eye ? What are they called ? How many fluids has the eye, and what are 
they called ? What is the lens of the eye called ? What coat forms the 
white of the eye ? Describe where the several coats and humors are situated. 
What is the ins ? What is the retina ? 









VISION. 


213 

posterior chamber of the eye, which is filled with the vitreous 
humor, v , v This humor occupies much the largest por¬ 
tion of the whole eye, and on it depends the shape and per¬ 
manency of the organ. 

727. From the above description of the eye, it will be 
easy to trace the progress of the rays of light through its 
several parts, and to explain in what manner vision is per¬ 
formed. 

In doing this, we must keep in mind that the rays of light 
proceed from every part and point of a visible object, as 
heretofore stated, and that it is necessary only for a few of 
the rays, when compared with the whole number, to enter 
the eye, in order to make the object visible. 

Thus, the object a, b, 
fig. 178, being placed in 
the light, sends forth pen¬ 
cils of rays in all possible 
directions, some of which 
will strike the eye in any 
position where it is visible. 

These pencils of rays not 
only flow from the points 
designated in the figure, 
but in the same manner 
from every other point on 
the surface of a visible 
object. To render an 
object visible, therefore, it 
is only necessary that the 
eye should collect and 
concentrate a sufficient 
number of these rays on 
the retina, to form its image there, and from this image the 
sensation of vision is excited. 

728. From the luminous body l , fig. 179, the pencils of 
rays flow in all directions, but it is only by those which en¬ 
ter the pupil, that we gain any knowledge of its existence ; 
and even these would convey to the mind no distinct idea of 
the object, unless they were refracted by the humors of the 
eye, for did these rays proceed in their natural state of di¬ 
vergence to the retina, the image there formed would be too 


Where is the sense of vision? What is the design of fig. 178? What is 
said concerning the small number of the rays which enter the eye from a visi 
hie object? Explain the design of fig. 179 








214 


VISION. 


exter sive, and consequently too feeble to give a distinct sen- 
satioi 1 of the object. 

Fig. 179. 



it is, therefore, by the refracting power of the aqueous 
humor, and of the crystalline lens, that the pencils of rays 
are so concentrated as to form a perfect picture of the object 
on the retina. 

Inverted Image on the Retina .—We have already seen, that 
when the rays of light are made to cross each other by re¬ 
flection from the concave mirror, the image of the object is 
inverted; the same happens when the rays are made to cross 
each other by refraction through a convex lens. This, in¬ 
deed, must be a necessary consequence of the intersection of 
the rays: for as light proceeds in straight lines, those rays 
which come from the lower part of an object, on crossing 
those which come from its upper part, will represent this part 
of the picture on the upper half of the retina, and, for the 
same reason, the upper part of the object will be painted on 
the lower part of the retina. 

729. Now, all objects are represented on the retina in an 
inverted position; that is, what we call the upper end of a 
vertical object, is the lower end of its picture on the retina, 
and so the contrary. 

Eye of an Ox .—This is readily proved by taking the eye 
of an ox, and cutting away the sclerotic coat, so as to make 
it transparent on the back part, next the vitreous humoi 
If now a piece of white paper be placed on this part of the 
eye, the images of objects will appear figured on the paper 
in an inverted position. The same effect will be produced on 
looking at things through an eye thus prepared ; they will 
appear inverted. 


Why would not the rays of light give a distinct idea of the object, without 
refraction by the humors of the eye ? Explain how it is that the images of 
objects are inverted on the retina. What experiment proves that the images 
of objects are inverted on the retina? 




VISION. 


215 


The actual position of the vertical object a , fig. 180, 
as painted on the retina, is therefore such as is represented 
by the figure. 

The rays from Fig. 180. 

its upper ex¬ 
tremity, com¬ 
ing in diverg¬ 
ent lines, are 
converged by 
the crystalline 
lens, and fall 
on the retina at 
o; while those 

from its lower extremity, by the same law, fall on the retina 
at c. 

730. In order that vision may be perfect, it is necessary . 
that the images of objects should be formed precisely on the 
retina, and consequently, if the refractive power of the eye 
be too small, or too great, the image will not fall exactly on 
the seat of vision, but will be formed either before, or tend to 
form behind it. In both cases, perhaps, an outline of the 
object may be visible, but it will be confused and indistinct. 

731. If the cornea is too convex, or prominent, the image 
will be formed before it reaches the retina, for the same rea¬ 
son, that of two lenses, that which is most convex will have 
the least focal distance. Such is the defect in the eyes of 
persons who are short sighted, and hence the necessity of 
their bringing objects as near the eye as possible, so as to 
make the rays converge at the greatest distance behind the 
crystalline lens. 

The effect of uncommon convexity in the cornea on the 
rays of light, is shown at fig. 181, where it will be observed 










216 


VISION. 


that the image, instead of being formed on the retina r, is 
suspended in the vitreous humor, in consequence of there be¬ 
ing too great a refractive power in the eye. It is hardly ne¬ 
cessary to say, that in this case, vision must be very imper¬ 
fectly performed. 

This defect of sight is remedied by spectacles, the glasses 
of which are concave lenses. Such glasses, by rendering 
the rays of light less convergent, before they reach the eye, 
counteract the too great convergent power of the cornea and 
lens, and thus throw the image on the retina. 

732. If, on the contrary, the humors of the eye, in conse¬ 
quence of age, or any other cause, have become less in quan¬ 
tity than ordinary, the eyeball will not be sufficiently dis¬ 
tended, and the cornea will become too flat, or not sufficiently 
convex, to make the rays of light meet at the proper place, 
and the image will therefore tend to be formed beyond the 
retina, instead of before it, as in the other case. Hence, aged 
people, who labor under this defect of vision, cannot see dis¬ 
tinctly at ordinary distances, but are obliged to remove the 
object as far from the eye as possible, so as to make its re¬ 
fractive power bring the image within the seat of vision. 

The defect arising from this cause is represented by fig. 
182, where it will be observed that the image is formed be- 


Fig. 182. 



hind the retina, showing that the convexity of the cornea is 
not sufficient to bring the image within the seat of distinct 
vision. This imperfection of sight is common to aged per¬ 
sons, and is corrected in a greater or less degree by double 
convex lenses, such as the common spectacle glasses. Such 

How is the sight improved, when the cornea is too convex ? How do such 
lenses act to improve the sight? Where do the rays tend to meet when the 
cornea is not sufficiently convex ? How is vision assisted when the eye wants 
convexity ? How do convex lenses help the sight of aged persons ? 




VISION 


217 


glasses, by causing the rays of light to converge, before they 
meet the eye, assist the refractive power of the crystalline 
lens, and thus bring the focus, or image, within the sphere 
of vision. 

733. Why we see objects erect .—It has been considered dif¬ 
ficult to account for the reason why we see objects erect, 
when they are painted on the retina inverted, and many 
learned theories have been written to explain this fact. But 
it is most probable that this is owing to habit, and that the 
image, at the bottom of the eye, has no relation to the terms 
above and below, but to the position of our bodies, and other 
things which surround us. The term perpendicular, and the 
idea which it conveys to the mind, is merely relative; but 
when applied to an object supported by the earth, and extend¬ 
ing towards the skies, we call the body erect, because it co¬ 
incides with the position of our own bodies, and we see it 
erect for the same reason. Had we been taught to read 
by turning our books upside down, what we now call the 
upper part of the book would have been its under part, and 
that reading would have been as easy in that position as in 
any other, is plain from the fact that printers read their types, 
when set up, as readily as they do its impressions on paper. 

734. Angle of Vision .—The angle under which the rays 
of light, coming from the extremities of an object, cross each 
other at the eye, bears a proportion directly to the length, 
and inversely to the distance of the object. 

Suppose the object a b , fig. 183, to be four feet long, and 
to be placed ten feet from the eye, then the rays flowing 
from its extremities, would intersect each other at the eye, 


Fig. 183. 



Why do we see things erect, when the images are inverted on the retina ? 
What is the visual angle ? How may the visual angle of the same object be 
increased or diminished ? When do objects of different magnitudes form the 
same visual angle ? Explain fig. 183. 







215 


VISION. 


under a given angle, which will always be the same when 
the object is at the same distance. If the object be gradually 
moved towards the eye, to the place c d, then the angle will 
be gradually increased in quantity, and the object will appear 
larger, since its image on the retina will be increased in 
length in the proportion as the lines i i are wider apart than 
o o. On the contrary, were a b removed to a greater distance 
from the first position, it is obvious that the angle would be 
diminished in proportion. 

The lines thus proceeding from the extremities of an ob¬ 
ject, and representing the rays of light, form an angle at the 
eye, which is called the visual angle , or the angle under 
which things are seen. These lines a n b, therefore, form 
one visual angle, and the lines end another visual angle. 

We see from this investigation, that the apparent magni¬ 
tude of objects depending on the angles of vision, will vary 
according to their distances from the eye, and that these 
magnitudes diminish in a proportion inversely as their dis¬ 
tances increase. We learn, also, from the same principles, 
that objects of different magnitudes may be so placed, with 
respect to the eye, as to give the same visual angle, and thus 
to make their apparent magnitudes equal. Thus the three 
arrows a , e, and m, though differing so much in length are 
all seen under the same visual angle. 

735. How we judge of Magnitudes .—In the apparent mag¬ 
nitude of objects seen through a lens, or when their images 
reach the eye by reflection from a mirror, our senses are 
chiefly, if not entirely, guided by the angle of vision. In 
forming our judgment of the sizes of distant objects, whose 
magnitudes were before unknown, we are also guided more 
or less by the visual angle, though in this case we do not 
depend entirely on the sense of vision. Thus, if we see two 
balloons floating in the air, one of which is larger than the 
other, we judge of their comparative magnitudes by the dif¬ 
ference in their visual angle,, and of their real magnitudes by 
the same angles, and the distance we suppose them to be 
from us. 

But when the object is near us, and seen with the nalted 
eye, we then judge of the magnitude by our experience, and 
not entirely by the visual angle. Thus, the three arrows, 
a, e , m, fig. 183, all of them make the same angle on the 


Under what circumstances is our sense of vision guided entirely by the vis 
ual angle ? How do we judge of the magnitudes of distant objects ? How do 
we judge of the comparative size of objects near us ? 



VISION. 


219 


eye, and yet we know, by further examination, that they aro 
all of different lengths. And so the two arrows a 6, and c c/, 
though seen under different visual angles, will appear of the 
same size, because experience has taught us that this d:5- 
ference depends only on the comparative distance of the two 
objects. 

736. As the visual angle diminishes inversely in propor¬ 
tion as the distance of the object increases, so when the dis¬ 
tance is so great as to make the angle too minute to be per¬ 
ceptible to the eye, then the object becomes invisible. Thus, 
when we watch an eagle flying from us, the angle of vision 
is gradually diminished, until the rays proceeding from the 
bird form an image on the retina too small to excite sensa¬ 
tion, and then we say the eagle has flown out of sight. 

The same principle holds with respect to objects which 
are near the eye, but are too small to form an image on the 
retina which is perceptible to the senses. Such objects to 
the naked eye, are of course invisible, but when the visual 
angle is enlarged, by means of the convex lens, they become 
visible; that is, their images on the retina excite sensation. 

737. Size of the Image on the Retina .—The actual size 
of an image on the retina, capable of exciting sensation, and 
consequently of producing vision, may be too small for us to 
appreciate by any of our other senses ; for when we consider 
how much smaller the image must be than the object, and 
that a human hair can be distinguished by the naked eye at 
the distance of twenty or thirty feet, we must suppose that 
the retina is endowed with the most delicate sensibility, to be 
excited by a cause so minute. It has been estimated that 
the image of a man, on the retina, seen at the distance of a 
mile, is not more than the five-thousandth part of an inch in 
length. 

738. Indistinct Vision .—On the contrary, if the object be 
brought too near the eye, its image becomes confused and 
indistinct, because the rays flowing from it, fall on the crys¬ 
talline lens in a state too divergent to be refracted to a focus 
on the retina. 

This will be apparent by fig. 184, where we suppose that 
the object a, is brought within an inch or two of the eye, 
and that the rays proceeding from it enter the pupil so 


When does a retreating object become invisible to the eye ? How does a 
convex lens act to make us see objects which are invisible without it ? What 
is said of the actual size of an image on the retina? Why are objects indis 
tinct, when brought too near the eye ? 



220 


MICROSCOPE. 



obliquely as not to be Fig. 184 - 

refracted by the lens, so 
as to form a distinct 
image. 

Could we see objects 
distinctly at the shortest 
distance, we should be 
able to examine things 
that are now invisible, 
since the visual angle 
would then be increas¬ 
ed, and consequently the image on the retina enlarged, in 
proportion as objects were brought near the eye. 

This is proved by intercepting the most divergent rays; 
in which case an object may be brought near the eye, and 
will then appear greatly magnified. Make a small orifice, 
as a pin-hole, through a piece of dark colored paper, and 
then look through the orifice at small objects, such as the 
letters of a printed book. The letters will appear much 
magnified. The rays, in this case, are refracted to a focus, 
on the retina, because the small orifice prevents those which 
are most divergent from entering the eye, so that notwith¬ 
standing the nearness of the object, the rays which form the 
image are nearly parallel. 

OPTICAL INSTRUMENTS. 

739. Single Microscope .—The principle of the single 
microscope, or convex lens, will be readily understood, if the 
pupil will remember what has been said on the refraction of 
lenses, in connection with the facts just stated. For, the 
reason why objects appear magnified through a convex lens, 
is not only because the visual angle is increased, but because 
when brought near the eye, the divergi.\graysfromtheobject 
are rendered parallel by the lens, and ais thus thrown into a 
condition to be brought to a focus in the proper place by the 
humors. 

Let a, fig. Fi g- 185 - 

185, be the 


b } thi 
tance 










MICROSCOPE. 


221 


which the same object is seen through the lens, and suppose 
the distance of a from the eye, be twice that of b. Then, 
because the object is at half the distance that it was before, 
it will appear twice as large ; and had it been seen one-third, 
one-fourth, or one-tenth its former distance, it would have 
been magnified three, four, or ten times, and consequently 
its surface would be increased 9, 16, or 100 times. 

740. The most powerful single microscopes are made of 
minute globules of glass, which are formed by melting the 
ends of a few threads of spun glass in a flame of alcohol. 
Small globules of water placed in an orifice through a piece 
of tin, or other thin substance, will also make very powerful 
microscopes. In these minute lenses, the focal distance is 
only a tenth or twelfth part of an inch from the lens, and 
therefore the eye, as well as the object to be magnified, must 
be brought very near the instrument. 

741. The Compound Microscope consists of two convex 
lenses, by one of which the image is formed within the tube 
of the instrument, and by the other this image is magnified, 
as seen by the eye; so that by this instrument the object 
itself is not seen, as with the single microscope, but we see 
only its magnified image. 

The small lens placed near the object, and by which its 
image is formed within the tube, is called the object glass , 
while the larger one, through which the image is seen, is 
called the eye glass. 

Fig. 186. 



Suppose objects could be seen distinctly within an inch or two of the eye, 
how would their dimensions be affected 1 How is it proved that objects 
placed near the eye are magnified ? How does a small orifice enable us to 
see an object distinctly near the eye ? Why does a convex lens make an 
object distinct when near the eye ? Explain fig. 185. How are the most pow¬ 
erful single microscopes made ? How many lenses form the compound micro¬ 
scope ? Which is the object, and which the eye glass ? Is the object seen 
with this instrument, or only its image ? Explain fig. 186, and show where the 
image is formed in this tube. 

19 * 









222 


MICROSCOPE. 


This arrangement is represented at fig. 186. The object 
a is placed a little beyond the focus of the object glass b } by 
which an inverted and enlarged image of it is formed within 
the instrument at c. This image is seen through the eye 
glass d, by which it is again magnified, and it is at last 
figured on the retina in its original position. 

These glasses are set in a case of brass, the object glass 
being made to take out, so that others of different magnify¬ 
ing powers may be used, as occasion requires. 

742. The Solar Microscope consists of two lenses, one of 
which is called the condenser , because it is employed to con¬ 
centrate the rays of the sun, in order to illuminate more 
strongly the object to be magnified. The other is a double 
convex lens, of considerable magnifying power, by which 
the image is formed. In addition to these lenses, there is a 
plain mirror, or piece of common looking glass, which can 
be moved in any direction, and which reflects the rays of the 
sun on the condenser. 

The object o, fig. 187, being placed nearly in the focus of 
the condenser b , is strongly illuminated, in consequence of 


o 0 Fig. l 87 



the rays of the sun being thrown on 5, by the mirror c. The 
object is not placed exactly in the focus of the condenser, be¬ 
cause, in most cases, it would be soon destroyed by its heat, 
and because the focal point would illuminate only a small 
extent of surface, but may be exactly in the focus of the 
small lens d , by which no such accident can happen. The 
lines o o , represent the incident rays of the sun, which are 
reflected on the condenser. 

When the solar microscope is used, the room is darkened, 
the only light admitted being that which is thrown on the 

How many lenses has the solar microscope ? Why is one of the lenses of 
the solar microscope called the condenser? Describe the uses of the two 
lenses and the reflector. 














TELESCOPE. 


223 


object by the condenser, which light passing through the 
small lens, gives the magnified shadow e, of the small ob¬ 
ject «, on the wall of the room, or on a screen. The tube 
containing the two lenses is passed through the window of 
the room, the reflector remaining outside. 

In the ordinary use of this instrument, the object itself is 
not seen, but only its shadow on the screen, and it is not de¬ 
signed for the examination of opaque objects. 

743. When the small lens of the solar microscope is of 
great magnifying power, it presents some of the most striking 
and curious of optical phenomena. The shadows of mites 
from cheese, or figs, appear nearly two feet in length, pre¬ 
senting an appearance exceedingly formidable and disgust¬ 
ing ; and the insects from common vinegar appear eight or 
ten feet long, and in perpetual motion, resembling so many 
huge serpents. 


TELESCOPE. 

744. The Telescope is an optical instrument , employed to 
view distant bodies, and , in effect , to bring them nearer the eye , 
by increasing the apparent angles under which such objects are 
seen. 

These instruments are of two kinds, namely, refracting 
and reflecting telescopes. In the first kind, the image of the 
object is seen with the eye directed towards it; in the second 
kind, the image is seen by reflection from a mirror, while the 
back is towards the object, or by a double reflection, with 
the face towards the object. 

The telescope is the most important of all optical instru¬ 
ments, since it unfolds the wonders of other worlds, and 
gives us the means of calculating the distances of the heav¬ 
enly bodies, and of explaining their phenomena for astrono¬ 
mical and nautical purposes. 

The principle of the telescope will be readily compre¬ 
hended after what has been said concerning the compound 
microscope, for the two instruments differ chiefly in respect to 
the place of the object lens, that of the microscope having 
a short, while that of the telescope has a long, focal distance. 

745. Refracting Telescope. —The most simple refract¬ 
ing telescope consists of a tube, containing two convex lenses, 


Is the object, or only the shadow, seen by this instrument? What is a tel¬ 
escope ? How many kinds of telescopes are mentioned ? What is the differ¬ 
ence between them ? In what respect does the refracting telescope differ from 
the compound microscope ? 



2S4 


TELESCOPE. 


the one having a long, and the other a short, focal distance. 
(The focal distance of a double convex lens, it will be re* 
membered, is nearly the centre of the sphere, of which it is 
a part. 686.) These two lenses are placed in the tube, at a 
distance from each other equal to the sum of their two focal 
distances. 

Fig. 188 . 



Thus, if the focus of the object glass a, fig. 188, be eight 
inches, and that of the eye glass b , two inches, then the dis¬ 
tance of the sums of the foci will be ten inches, and, there¬ 
fore, the two lenses must be placed ten inches apart; and 
the same rule is observed, whatever may be the focal lengths 
of any two lenses. 

Now, to understand the effect of this arrangement, sup¬ 
pose the rays of light, c d , coming from a distant object, as 
a star, to fall on the object glass <z, in parallel lines, and to 
be refracted by the lens to a focus at <?, where the image of 
the star will be represented. The image is then magnified 
by the eye glass b, and thus, in effect, is brought near the 
eye. 

746. All that is effected by the telescope, therefore, is to 
form an image of a distant object, by means of the object 
lens, and then to assist the eye in viewing this image as 
nearly as possible by the eye lens. 

It is, however, necessary here to state, that by the last 
figure, the principle only of the telescope is intended to be 
explained, for in the common instrument, with only two 
glasses, the image appears to the eye inverted. 

The reason of this will be seen by the next figure, where 
the direction of the rays of light will show the position of 
the image. 

Suppose a, fig. 189, to be a distinct object, from which 
pencils of rays flow from every point toward the object lens 
b. The image of a , in consequence of the refraction of the 


How is the most simple refracting telescope formed? Which is the object, 
and which the eye lens, in fig. 188? What is the rule by which the distance of 
the two glasses apart is found ? How do the two glasses act, to bring an ob¬ 
ject near the eye ? 














TELESCOPE. 


225 


Fig. 189. 



ra.ys by the object lens, is inverted at c, which is the focus 
of the eye glass </, and through which the image is then 
seen, still inverted. 

747. The inversion of the object is of little consequence 
when the instrument is employed for astronomical purposes, 
for since the forms of the heavenly bodies are spherical, their 
positions, in this respect, do not affect their general appear¬ 
ance. But for terrestrial purposes, this is manifestly a great 
defect, and therefore those constructed for such purposes, as 
ship, or spy glasses, have two additional lenses, by means of 
which, the images are made to appear in the same position 
as the objects. These are called double telescopes. 



Such a telescope is represented at fig. 190, and consists of 
an object glass a , and three eye glasses, b , c, and d. The eye 
glasses are placed at equal distances from each other, so that 
the focus of one may meet that of the other, and thus the 
image formed by the object lens, will be transmitted through 
the other three lenses to the eye. The rays coming from the 
object o, cross each other at the focus of the object lens, and 
thus form an inverted image at f This image being also in 
the focus of the first eye glass, 6, the rays having passed 
through this glass become parallel, for we have seen in an¬ 
other place, that diverging rays are rendered parallel by re¬ 
fraction through a convex lens. The rays, therefore, pass 


Explain fig. 189, and show how the object comes to be inverted by the two 
lenses ? How is the inversion of the object corrected ? Explain fig. 190, and 
show why the two additional lenses make the image of the object erect. 








226 


TELESCOPE. 


parallel to the next lens c, by which they are made to con¬ 
verge, and cross each other, and thus the image is inverted, 
and made to assume the original position of the object o. 
Lastly, this image, being in the focus of the eye glass d, is 
seen in the natural position. 

The apparent magnitude of the object is not changed by 
these two additional glasses, but depends, as in fig. 188, on 
the magnifying power of the eye and object lenses ; the two 
glasses being added merely for the purpose of making the 
image appear erect. 

748. It is found that an eye glass of very high magnify¬ 
ing power cannot be employed in the refracting telescope, 
because it disperses the rays of light, so that the image be¬ 
comes ifidistinct. Many experiments were formerly made 
with a view to obviate this difficulty, and among these it 
was found that increasing the focal distance of the object 
lens, was the most efficacious. But this was attended with 
great inconvenience and expense, on account of the length 
of tube which this mode required. These experiments were, 
however, discontinued, and the refracting telescope itself 
chiefly laid aside for astronomical purposes, in consequence 
of the discovery of the reflecting telescope. 

749. Reflecting Telescope. —The common reflecting 
telescope consists of a large tube, containing two concave 
reflecting mirrors, of different sizes, and two eye glasses. 
The object is first nflected from the large mirror to the small 
one, and from the small one, through the two eye glasses, 
where it is then seen. 

750. In comparing the advantages of the two instruments, 
it need only be stated, that the refracting telescope, with a 
focal length of a thousand feet, if it could be used, would not 
magnify distinctly more than a thousand times, while a re¬ 
flecting telescope, only eight or nine feet long, will magnify 
with distinctness twelve hundred times. 

751. The principle and construction of the reflecting tele¬ 
scope will be understood by fig. 191. Suppose the object o 
to be at such a distance, that the rays of light from it pass 
in parallel lines, p, p, to the great reflector r, r. This reflector 
being concave, the rays are converged by reflection, and 
cross each other at a, by which the image is inverted. The 


Does the addition of these two lenses make any difference with the apparent 
magnitude of the object ? Why cannot a highly magnifying eye glass be used in 
this telescope ? What is the most efficacious means of increasing the power of 
the refracting telescope ? How many lenses and mirrors form the reflecting tele¬ 
scope ? What are the advantages of the reflecting over the refracting telescope ? 




TELESCOPE. 


227 


Fig. 191. 


r 



d e 



o 


r 


rays then pass to the small mirror, b , which being also con¬ 
cave, they are thrown back in nearly parallel lines, and hav¬ 
ing passed the aperture in the centre of the great mirror, fall 
on the plano-convex lens e. By this lens they are refracted 
to a focus, and cross each other between e and d : and thus 
the image is again inverted, and brought to its original po¬ 
sition, or in the position of the object. The rays then pass¬ 
ing the second eye glass, form the image of the object on the 
retina. 

The large mirror in this instrument is fixed, but the small 
one moves backwards and forwards, by means of a screw, 
so as to adjust the image to the eyes of different persons. 
Both mirrors are made of a composition, consisting of several 
metals melted together. 

752. One great advantage which the reflecting telescope 
possesses over the refracting, appears to be, that it admits of an 
eye glass of shorter focal distance, and, consequently, of great¬ 
er magnifying power. - The convex object glass of the re¬ 
fracting instrument, does not form a perfect image of the ob¬ 
ject, since some of the rays are dispersed, and others colored 
by refraction. This difficulty does not occur in the reflected 
image from the metallic mirror of the reflecting telescope, 
and consequently it may be distinctly seen", when more highly 
magnified. 

The instrument just described is called “ Gregory's tele¬ 
scope” because some parts of the arrangement were invented 
by Dr. Gregory. 

753. In the telescope made by Dr. Herschel, the object is 
reflected by a mirror, as in that of Dr. Gregory. But the 
second, or small reflector, is not employed, the image being 
seen through a convex lens, placed so as to magnify the im- 


Explain fig. 191, and show the course of the rays from the object to the eye. 
Why is the small mirror in this instrument made to move by means of a screw 
W hat is the advantage of the reflecting telescope in respect to the eye glass i 
Why is the telescope with two reflectors called Gregory’s telescope . 


























TELESCOPE. 


228 

age of'the large mirror, so that the observer stands with his 
back towards the object. 

The magnifying power of this instrument is the same as 
that of Dr. Gregory’s, but the image appears brighter, be¬ 
cause there is no second reflection ; for every reflection ren¬ 
ders the image fainter, since no mirror is so perfect as to 
throw back all the rays which fall upon its surface. 

754. In Dr. Herschel’s grand telescope, the largest ever 
constructed, the reflector was 48 inches in diameter, and had 
a focal distance of 40 feet. This reflector was three and a 
half inches thick, and weighed 2000 pounds. Now, since 
the focus of a concave mirror is at the distance of one-half 
the semi-diameter of the sphere, of which it is a section, 
Dr. Herschel’s reflector having a focal distance of 40 feet, 
formed a part of a sphere of 160 feet in diameter. 

This great instrument was begun in 1785, and finished* 
four years afterwards. The frame by which this wonder to 
all astronomers was supported, having decayed, it was taken 
down in 1822, and another of 20 feet focus, with a reflector 
of 18 inches in diameter, erected in its place, by Herschel’s 
son. 

The largest Herschel’s telescope now in existence is that 
of Greenwich observatory, in England. This has a con¬ 
cave reflector of 15 inches in diameter, with a focal length 
of 25 feet, and was erected in 1820. 

755. Camera Obsctjra. —Camera obscura strictly signi¬ 
fies a darkened el^amber, because the room must be dark¬ 
ened, in order to observe its effects. 

To witness the phenomena of this instrument, let a room 
be closed in every direction, so as to exclude the light. Then 
from an aperture, say of an inch in diameter, admit a single 
beam of light, and . the images of external things, such as 
trees, and houses, and persons walking the streets, will be 
seen inverted on the wall opposite to where the light is ad¬ 
mitted, or on a screen of white paper, placed before the aper¬ 
ture. 

756. The reason why the image is inverted, will be obvi¬ 
ous, when it is remembered that the rays proceeding from - 
the extremities of the object must converge in order to pass 

How does this instrument differ from Dr. Herschel’s telescope? What was 
the focal distance and diameter of the mirror in Dr. Herschel’s great telescope ? 
Where is the largest Herschel’s telescope now in existence ? What is the di¬ 
ameter and focal distance of the reflector of this telescope ? Describe the phe¬ 
nomena of the camera obscura. Why is the image formed by the camera ob¬ 
scura inverted ? 



CAMERA OBSCURA. 


229 


through the small aperture; and as the rays of light always 
proceed in straight lines, they must cross each other at the 
point of admission, as explained under the article Vision. 

Thus the pen¬ 
cil a, fig. 192, Fig. 192. 

coming from the 
upper part of the 
tower, and pro¬ 
ceeding straight 
-will represent 
the image of 
that part at 6, 
while the low¬ 
er part c, for the 
same reason, 
will be repre¬ 
sented at d. If a convex lens, with a short tube, be placed 
in the aperture through which the light passes into the room, 
the images of things will be much more perfect, and their 
colors more brilliant. 



Fig. 193. 


a 


757. This instrument is 
sometimes employed by 
painters, in order to obtain 
an exact delineation of a 
landscape, an outline of the 
image being easily taken 
with a pencil, when the im¬ 
age is thrown on a sheet of 
paper. 

There are several modi¬ 
fications of this machine, 
and among them the revolv¬ 
ing camera obscura is the 
most interesting. 

It consists of a small 
house, fig. 193, with a plane 
reflector a b , and a convex 
lens c b , placed at its top. 

The reflector is fixed at an angle of 45 degrees with the 
horizon so as to reflect the rays of light perpendicularly 
downwards, and is made to revolve quite arotmd, in either 
direction, by pulling a string. 

How may an outline of the image formed by the camera obscura be taken? 
Describe the revolving camera obscura 
20 





















230 


MAGIC LANTERN. 


Now suppose the small house to be placed in the open 
air, with the mirror, a b , turned towards the east, then the 
rays of light flowing from the objects in that direction, will 
strike the mirror in the direction of the lines o, and be re¬ 
flected down through the convex lens c b , to the table e e, 
where they will form in miniature a most perfect and beau¬ 
tiful picture of the landscape in that direction. Then, by 
making the reflector revolve, another portion of the land¬ 
scape may be seen, and thus the objects, in all directions, 
can be viewed at k without changing the place of the in¬ 
strument. 

758. Magic Lantern. — The magic lantern is a microscope, 
on the same principle as the solar microscope. —But instead of 
being used to magnify natural objects, it is commonly em¬ 
ployed for amusement, by the casting shadows of small 
transparent paintings done on glass, upon a screen placed at 
a proper distance. 



Fig. 194. 


Let a candle c, fig. 194, be placed on the inside of a box 
or tube, so that its light may pass through the plano-convex 
lens Ji. and strongly illuminate the object o . This object is 
generally a small transparent painting on a slip of glass, 
which slides through an opening in the tube. In order to 
show the figures in the erect position, these paintings are 
inverted, since their shadows are again inverted by the refrac¬ 
tion of the convex lens m. 

In some of these instruments, there is a concave mirror, d , 
by which the object o, is more strongly illuminated than it 
would be by the lamp alone. The object is magnified by the 


What is the magie lantern ? For what purpose is this instrument employed T 
Describe the construction and effect of the magic lantern. 
















CHROMATICS. 


231 

double convex lens, m, which is moveable in the tube by a 
screw, so that its focus can be adjusted to the required dis¬ 
tance. Lastly, there is a screen of white cloth, placed at 
the proper distance, on which the image or shadow of the 
picture, is seen greatly magnified. 

The pictures being of various colors, and so transparent, 
that the light of the lamp shines through them, the shadows 
are also of various colors, and thus soldiers and horsemen 
are represented in their proper costume. 

CHROMATICS, OR THE PHILOSOPHY OF COLORS. 

759. We have thus far considered light as a simple sub¬ 
stance, and have supposed that all its parts were equally 
refracted, in its passage, through the several lenses described. 
But it will now be shown that light is a compound body, 
and that each of its rays, which to us appear white, is 
composed of several colors, and that each color suffers a dif¬ 
ferent degree of refraction, when the rays of light pass 
through a piece of glass, of a certain shape. This was a 
discovery of Sir Isaac Newton’s. 

760. Solar Spectrum. —If a ray, proceeding from the 
sun, be admitted into a darkened chamber, through an aper¬ 
ture in the window shutter, and allowed to pass through a 
triangular shaped piece of glass, called a prism , the light will 
be decomposed, and instead of a spot of white, there will be 
seen, on the opposite wall, a most brilliant display of colors, 
including all those seen in a rainbow. 



Suppose 5 , fig. 195, to be a ray from the sun, admitted 

Who made the discovery, that light is a compound substance ? In what 

manner, and by what means, is light decomposed? What are the prismatic 
colors, and how do they succeed each other in the spectrum. 










282 


CHROMATICS. 


through the window shutter a , in such a direction as to fall 
on the floor at c, where it would form a round, white spot. 
Now, on interposing the prism />, the ray will be refracted, 
and at the same time decomposed, and will form on the 
screen m, n , an oblong figure, containing seven colors, which 
will be situated in respect to each other, as named in the 
figure. 

It may be observed, that of all the colors, the red is least 
refracted, or is thrown the smallest distance from the direc¬ 
tion of the original sun-beam, and that the violet is jnost re¬ 
fracted, or bent out of that direction. 

This oblong image containing the colored rays, is called 
the solar or prismatic spectrum. 

761. Recomposition of White Light. —That the rays of the 
sun are composed of the seven colors above named, is suffi¬ 
ciently evident by the fact, that such a ray is divided into 
these several colors by passing through the prism, but in ad¬ 
dition to this proof, it is found by experiment, that if these 
several colors be blended or mixed together, white will be the 
result. 

This may be done by mixing together seven powders, 
whose colors represent the prismatic colors, and whose quan¬ 
tities are to each other, as the spaces occupied by each color 
in the spectrum. When this is done, it will be found that 
the resulting color will be a grayish white. A still more 
satisfactory proof that these seven colors form white, when 
united, is obtained by causing the solar spectrum to pass 
through a Tens, by which they are brought to a focus, 
when it is found that the focus will be the same color as it 
would be from the original rays of the sun. 

762. From the oblong shape of the solar spectrum, we learn 
that each of the colored rays is refracted in a different degree 
by passing through the same medium, and consequently that 
each ray has a refractive power of its own. Thus, from the 
red to the violet, each ray, in succession, is refracted more 
than the other. 

763. Other means of Decomposing Light .—The prism is not 
the only instrument by which light can be decomposed. A - 
soap bubble blown up in the sun will display most of the 
prismatic colors. This is accounted for by supposing that 

Which color is refracted most and which least ? When the several pris¬ 
matic colors are blended, what color is the result? When the solar spectrum 
is made to pass through a lens, what is the color of the focus ? How do we 
learn that each colored ray has a refractive power of its own ? By what other 
means besides the prism, can the rays of light be decomposed ? 





RAINBOW. 


233 


the sides of the bubble vary in thickness, and that the rays 
of light are decomposed by these variations. Trie unequal 
surface of mother of 'pearl , and many other shells, send forth 
colored rays on the same principle. 

764. Two surfaces of polished glass, when pressed together 
will also decompose the light. Rings of colored light will 
be observed around the point of contact between the two sur¬ 
faces, and their number may be increased or diminished by 
the degrees of pressure. Two pieces of common looking- 
glass, pressed together with the fingers, will display most of 
the prismatic colors. 

765. A variety of substances, when thrown into the form 
of the triangular prism, will decompose the rays of light, as 
well as a prism of glass. A very common instrument for 
this purpose is made by putting together three pieces of plate 
glass, in form of a prism. The ends may be made of wood, 
and the edges cemented with putty, so as to make the whole 
water tight. When this is filled •with water, and held be¬ 
fore a sun-beam, the solar spectrum will be formed, display¬ 
ing the same colors, and in the same order, as that above 
described. 

766. In making experiments with prisms, filled with differ¬ 
ent kinds of liquids, it has been found that one liquid will 
make the spectrum longer than another ; that is, the red and 
violet rays, which form the extremes of the spectrum, will 
be thrown farther apart by one fluid than by another. For 
example, if the prism be filled with oil of cassia, the spec¬ 
trum formed by it, will be more than twice as long as that 
formed by a prism of solid glass. The oil of cassia is therefore 
said to disperse the rays of light more than glass, and hence 
to have a greater dispersive power. 

767. The Rainbow.— The rainbow was a phenomenon, 
for which the ancients were entirely unable to account; but 
after the discovery that light is a compound principle, and 
that its colors may be separated by various substances, the 
solution of this phenomenon became easy. 

Sir Isaac Newton, after his great discovery of the com¬ 
pound nature of light, and the different refrangibility of the 
colored rays, was able to explain the rainbow on optical 
principles. 

How may light be decomposed by two pieces of glass ? Of what substances 
may prisms be formed, besides glass ? What is said of some liquids making 
the spectrum larger than others ? What is said of oil of cassia, in this respect ? 
W hat discovery preceded the explanation of the rainbow ? Who first explained 
the rainbow on optical principles ? 

20* 



234 


RAINBOW. 


708. If a glass globe be suspended in a room, where the 
rays of the sun can fall upon it, the light will be decom¬ 
posed, or separated into several colored rajs, in the same 
manner as is done bj the prism. A well defined spectrum 
will not, however, be formed by the globe, because its shape 
is such as to disperse some of the rays, and converge others ; 
but the eye, by taking different positions in respect to the 
globe, will observe the various prismatic colors. Transpa¬ 
rent bodies, such as glass and water, reflect the rays of light 
from both their surfaces, but chiefly from the second surface. 
That is, if a plate of naked glass be placed so as to reflect 
the image of the sun, or of a lamp, to the eye, the most dis¬ 
tinct image will come from the second surface, or that most 
distant from the eye. The great brilliancy of the diamond 
is owing to this cause. It will be understood directly, how 
this principle applies to the explanation of the rainbow. 

How the Bow is 
formed .--Suppose 
the circle a b c, 
fig. 196, to repre¬ 
sent a globe, or 
a drop of rain, 
for each drop of 
rain, as it falls 
through the air, is 
a small globe of 
water. Suppose, 
also, that the sun 
is at 5 , and the 
eye of the specta¬ 
tor at e. Now, it has already been stated, (768) that from a 
single globe, the whole solar spectrum is not seen in the 
same position, but that the different colors are seen from differ¬ 
ent places. Suppose, then, that a ray of light from the sun 
on entering the globe at a, is separated into its primary colors, 
and at the same time the red ray, which is the least refran¬ 
gible, is refracted in the line from a to b. From the second, 
or inner surface of the drop, it would be reflected to c, the 
angle of reflection being equal to that of incidence. On 
passing out of the drop, its refraction at c, would be just 
equal to the refraction of the incident ray at «, and therefore 



Why does not a glass ^lobe form a well refined spectrum ? From which 
surface do transparent bodies chiefly reflect the light ? Explain fig. 196, and 
show the different refractions, and the reflection concerned in forming the rain 
bow. In the case supposed, why will only the red ray meet the eye \ 



RAINBOW. 


235 


the red raj would fall on the eye at e. All the other colored 
rays would follow the same law, but because the angles of 
incidence and those of reflection are equal, and because the 
colored rays are separated from each other by unequal re¬ 
fraction, it is obvious, that if the red ray entered the eye at e, 
none of the other colored rays could be seen from the same 
point. 

769. From this it is evident, that if the eye of the spec¬ 
tator is moved to another position, he will not see the red ray 
coming from the same drop of rain, but only the blue, and if 
to another position, the green, and so of all the others. But 
in a shower of rain, there are drops at all heights and dis¬ 
tances, and though they perpetually change their places, in 
respect to the sun and the eye, as they fall, still there will be 
many which will be in such a position as to reflect the red 
rays to the eye, and as many more to reflect the yellow rays, 
and so of all the other colors. 

This will be 
made obvious 
by figure 197, 
where, to avoid 
confusion, we 
will suppose 
that only three 
drops of rain, 
and, conse- ^ 
quently, only ^ 
three colors, are 
to be seen. c 

The numbers 
1,2, 3, are the 
rays of the sun, 
proceeding to 
the drops a, b, 
c, and from 
which these 
rays are reflect¬ 
ed to the eye, making different angles with the horizontal 
line A, because one colored ray is refracted more than another. 
Now, suppose the red ray only reaches the eye from the drop 
a, the green from the drop Z>, and the violet from the drop c, 



Suppose a person looking at a rainbow moves his eye, will he see the same 
colors from the same drop of rain? Explain fig. 197, and show why we see 
different colors from different drops of rain. 




236 


RAINBOW 


then the spectator would see a minute rainbow of three 
colors. But during a shower of rain, all the drops which are 
in the position of a, in respect to the eye, would send forth 
red rays, and no other, while those in the position of would 
emit green rays, and no other, and those in the position of c , 
violet rays, and so of all the other prismatic colors. Each 
circle of colors, of which the rainbow is formed, is therefore 
composed of reflections from a vast number of different drops 
of rain, and the reason why these colors are distinct to oui 
senses, is, that we see only one color from a single drop, with 
the eye in the same position. It follows, then, that if we 
change our position, while looking at a rainbow, we still see 
a bow, but not the same as before, and hence, if there »re 
many spectators, they will all see a different rainbow, thou, h 
it appears to be the same. 

770. Secondary Bow .—There are often seen two rainbows, 
the one formed as above described, and the other, which is 
fainter, appearing on the outside, or above this. The sec¬ 
ondary bow, as this last is called, always has its order of 
colors the reverse of the primary one. Thus, the colors of the 
primary bow, beginning with its upper or outermost portion, 
are red, orange, yellow, &c., the lowest or innermost portion, 
being violet; while the secondary bow, beginning with the 
same corresponding part, is colored violet, indigo, &c., the 
lowest, or innermost circle, being red. 

771. In the primary bow, we have seen, that the colored 
rays arrive at the eye after two refractions, and one reflec¬ 
tion. In the secondary bow, the rays reach the eye after 
two refractions, and two reflections, and the order of the 
colors is reversed, because, in this case, the rays of light enter 
the lower part of the drop, instead of the upper part, as in 
the primary bow. The reason why the colors are fainter in 
the secondary than in the primary bow is, because a part of 
the light is lost or dispersed, at each reflection, and there 
being two reflections, by which this bow is formed, instead of 
one, as in the primary, the difference in brilliancy is very 
obvious. 

772. The direction of a single ray, showing how the 
secondary bow is formed, will be seen at figure 198. The 


Do several persons see the same rainbow at the same time ? Explain the 
reason of this. How are the colors of the primary and secondary bows ar¬ 
ranged in respect to each other? How many refractions and reflections pro¬ 
duce the secondary bow ? Why is the secondary bow less brilliant than the 
primarv ? 



COLORS. 


237 


ray r, from Fig. 198. 



the eye of the spectator at e. 

The rainbow, being the consequence of the refracted and 
reflected rays of the sun, is never seen, except when the sun 
and the spectator are in similar directions in respect to the 
shower. It assumes the form of a semicircle, because it is 
only at certain angles that the refracted rays are visible to 
the eye. 

773. Colors of objects .—The light of the sun, we have 
seen, may be separated into seven primary rays, each of 
which has a color of its own, and which is different from 
that of the others. In the objects which surround us, both 
natural and artificial, we observe a great variety of colors, 
which differ from those composing the solar spectrum, 
and hence one might be led to believe that both nature and 
art afford colors different from those afforded by the decom¬ 
position of the solar rays. But it must be remembered, that 
the solar spectrum contains only the 'primary colors of na¬ 
ture, and that by mixing these colors in various proportions 
with each other, an indefinite variety of tints, all differing 
from their primaries, may be obtained. 

774. Color depends on absorption and reflection .—It ap¬ 
pears that the colors of all bodies depend on some peculiar 
property of their surfaces, in consequence of which, they ab¬ 
sorb some of the colored rays, and reflect the others. Had 
the surfaces of all bodies the property of reflecting the same 
ray only, all nature would display the monotony of a single 
color, and our senses would never have known the charms 
of that variety which we now behold. 

Why are the colors of things different from those of the solar spectrum ? 
On what do the colors of bodies depend ? Suppose all bodies reflected the 
same ray, what would be the consequence in regard to color? 






238 


COLORS. 


775. All bodies appear of the color of that ray, or of a tint 
depending on the several rays which it reflects, while all the 
other rays are absorbed, or, in other terms, are not reflected. 
Black and white, therefore, in a philosophical sense, cannot 
be considered as colors, since the first arises from the absorp¬ 
tion of all the rays, and the reflection of none, and the last is 
produced by the reflection of all the rays, and the absorption 
of none. But in all colors, or shades of color, the rays only 
are reflected, of which the color is composed. Thus, the 
color of grass, and the leaves of plants, is green, because the 
surfaces of these substances reflect only the green rays, and 
absorb all the others. For the same reason, the rose is red, 
the violet blue, and so of all other substances, every one 
throwing out the ray of its own color, and absorbing all the 
others. 

776. To account for such a variety of colors as we see in 
different bodies, it is supposed that all substances, when made 
sufficiently thin, are transparent, and consequently, that they 
transmit through their surfaces, or absorb, certain rays of 
light, while other rays are thrown back, or reflected, as 
above described. Gold, for example, may be beat so thin as 
to transmit some of the rays of light, and the same is true of 
several of the other metals, which are capable of being ham¬ 
mered into thin leaves. It is therefore most probable, that 
all the metals, could they be made sufficiently thin, would 
permit the rays of light to pass through them. Most, if not 
quite all mineral substances, though in the mass they may 
seem quite opaque, admit the light through their edges, 
when broken, and almost every kind of wood, when made 
no thinner than writing paper, becomes translucent. Thus 
we may safely conclude, that every substance with which 
we are acquainted, will admit the rays of light, when made 
sufficiently thin. 

777. Transparent colorless substances, whether solid or 
fluid, such as glass, water, or mica, reflect and transmit light 
of the same color; that is, the light seen through these 
bodies, and reflected from their surfaces, is white. This is 
true of all transparent substances under ordinary circum¬ 
stances ; but if their thickness be diminished to a certain 
extent, these substances will both reflect and transmit 


Why are not black and white considered as colors ? Why is the color of 
grass green? How is the variety of colors accounted for, by considering all 
bodies transparent ? What is said of the reflection of colored light by trans¬ 
parent substances ? What substance is mentioned, as illustrating this fact ? 



COLORS. 


239 


colored light of various hues, according to their thickness. 
Thus, the thin plates of mica, which are left on the fingers 
after handling that substance will reflect prismatic rays of 
various colors. 

778. There is a degree of tenuity, at which transparent 
substances cease to reflect any of the colored rays, but ab¬ 
sorb, or transmit them all, in which case they become black. 
This may be proved by various experiments. If a soap bub¬ 
ble be closely observed, it will be seen that at first, the thick¬ 
ness is sufficient to reflect the prismatic rays from all its parts, 
but as it grows thinner, and just before it bursts, there may 
be seen a spot on its top, which turns black, thus transmit¬ 
ting all the rays at that part, and reflecting none. The same 
phenomenon is exhibited, when a film of air, or water, is 
pressed between two plates of glass. At the point of con¬ 
tact, or where the two plates press each other with the 
greatest force, there will be a black spot, while around this 
there may be seen a system of colored rings. 

From such experiments, Sir Isaac Newton concluded, that 
air, when below the thickness of half a millionth of an inch , 
ceases to reflect light; and also that water, when below the 
thickness of three eighths of a millionth of an inch , ceases to 
reflect light. But that both air and water, when their thick¬ 
ness is in a certain degree above these limits, reflect all the 
colored rays of the spectrum. 

779. Now all solid bodies are more or less porous, having 
among their particles either void spaces, or spaces filled with 
some foreign matter, differing in density from the body itself, 
such as air or water. Even gold is not perfectly compact, 
since water can be forced through its pores. It is most proba¬ 
ble, then, that the parts of the same body, differing in densi¬ 
ty, either reflect, or transmit the rays of light, according to 
the size or arrangement of their particles; and in proof of 
this, it is found that some bodies transmit the rays of one color, 
and reflect that of another. Thus, the color which passes 
through a leaf of gold is green, while that which it reflects 
is yellow. 

780. From a great variety of experiments on this subject, 
Sir Isaac Newton concludes that the transparent parts of 
bodies, according to the sizes of their transparent pores, re- 

When is it said that transparent substances become black ? How is it 
proved that fluids of extreme tenuity absorb all the rays and reflect none ? 
What is the conclusion of Sir Isaac Newton, concerning the tenuity at which 
water and air ceases to reflect light ? What is said of the porous nature of the 
solid bodies ? 



240 


ASTRONOMY. 


fleet rays of one color, and transmit those of another, for the 
same reason that thin plates, or minute particles of air, water, 
and some other substances, reflect certain rays, and absorb, 
or transmit others, and that this is the cause of all their colors. 

*81. In confirmation of the truth of this theory, it may be 
observed, that many substances, otherwise opaque, become 
transparent, by filling their pores with some transparent fluid. 

Thus, the stone called Hydrophane, is perfectly opaque 
when dry, but becomes transparent when dipped in water; 
and common writing paper becomes translucent, after it has 
absorbed a quantity of oil. The transparency, in these cases, 
may be accounted for, by the different refractive powers 
which the water and oil possess, from the stone or paper, and 
in consequence of which the light is enabled to pass among 
their particles by refraction. 


ASTRONOMY. 

782. Astronomy is that science which treats of the motions 
and appearances of the heavenly bodies ; accounts for the phe¬ 
nomena which these bodies exhibit to us; and explains the laws 
by which their motions , or apparent motions , are regulated. 

Astronomy is divided into Descriptive , Physical , and Prac¬ 
tical. 

Descriptive astronomy demonstrates the magnitudes, dis¬ 
tances, and densities of the heavenly bodies, and explains the 
phenomena dependent on their motions, such as the change 
of seasons, and the vicissitudes of day and night. 

Physical astronomy explains the theory of planetary mo¬ 
tion, and the laws by which this motion is regulated and 
sustained. 

Practical astronomy details the description and use of as¬ 
tronomical instruments, and developes the nature and appli¬ 
cation of astronomical calculations. 

The heavenly bodies are divided into three distinct classes, 
or systems, namely, the solar system , consisting of the sun, 
moon, and planets, the system of the fixed stars, and the sys¬ 
tem of the comets. 


What is astronomy ? How is astronomy divided ? What does descriptive 
astronomy teach? What is the object of physical astronomy ? What is prac¬ 
tical astronomy ? How are the heavenly bodies divided ? Of what does the 
solar system consist ? What are the bodies called, which revolve around the 
Sun as a centre ? 




ASTRONOMY. 


21 ! 


THE SOLAR SYSTEM. 

783. The Solar System consists of the Sun , and twenty-nine 
other bodies , which revolve around him at various distances , and 
in various periods of time. 

The bodies which revolve around the Sun as a centre, are 
called primary planets. Thus, the Earth, Venus, and Mars, 
are primary planets. Those which revolve around the prima¬ 
ry planets, are called secondary planets, Moons , or Satellites. 
Our Moon is a secondary planet or satellite. 

The primary planets revolve around the Sun in the follow¬ 
ing order, and complete their revolutions in the following 
times, computed in our days and years. Beginning with 
that nearest to the Sun, Mercury performs his revolution in 
87 days and 23 hours; Venus, in 224 days, 17 hours ; the 
Earth, attended by the Moon, in 365 days, 6 hours; Mars, 
in one year, 322 days ; Ceres, in 4 years, 7 months, and 10 
days ; Pallas, in 4 years, 7 months, and 10 days ; Juno, in 
4 years and 128 days; Vesta, in 3 years, 66 days and 4 
hours; Jupiter, in 11 years, 315 days and 15 hours; Sat¬ 
urn, in 29 years, 161 days and 19 hours ; He^schel, in 83 
years, 342 days and 4 hours. 

784. A year consists of the time which it takes a planet 
to perform one complete revolution through its orbit, or to 
pass once around the Sun. Our Earth performs this rev¬ 
olution in 365 days, and therefore this is the period of our 
year. Mercury completes his revolution in 88 days, and 
therefore his year is no longer than 88 of our days. But the 
planet Herschel is situated at such a distance from the Sun, 
that his revolution is not completed in less than about 84 of 
our years. The other planets complete their revolutions in 
various periods of time, between these; so that the time of 
these periods is generally in proportion to the distance of 
each planet from the Sun. 

Ceres, Pallas, Juno, and Vesta, are the smallest of all the 
planets, and are called Asteroids. 

Besides the above enumerated primary planets, our sys¬ 
tem contains eighteen secondary planets, or moons. Of 
these, our Earth has one moon, Jupiter four, Saturn seven. 

What are those called which revolve around these primaries as a centre T 
In what order are the several planets situated, in respect to the Sun ? How 
long does it take each planet to make its revolution around the Sun ? What 
is a year ? What planets are called asteroids ? How many moons does our 
system contain ? Which of the planets are attended by moons, and how many 
has each? 


21 




242 


ASTRONOMY. 


and Herschel six. None of these moons, except our own, 
and one or two of Saturn’s, can be seen without a telescope. 
Tiie seven other planets, so far as has been discovered, are 
entirely without moons. 

785. All the planets move around the Sun from west to 
east, and in the same direction do the moons revolve around 
their primaries, with the exception of those of Herschel, 
which appear to revolve in .a contrary direction. 

786. Orbits of the Planets .—The paths in which the plan¬ 
ets move round the Sun, and in which the moons move round 
their primaries, are called their orbits. These orbits are not 
exactly circular, as they are commonly represented on paper, 
but are elliptical, or oval, so that all the planets are nearer 
the Sun, when in one part of their orbits than when in 
another. 

In addition to their annual revolutions, some of the plan¬ 
ets are known to have diurnal, or daily revolutions, like our 
Earth. The periods of these daily revolutions have been as¬ 
certained, in several of the planets, by spots on their surfaces. 
But where no such mark is discernible, it cannot be ascer¬ 
tained whether the planet has a daily revolution or not, 
though this has been found to be the case in every instance 
where spots are seen, and, therefore, there is little doubt but 
all have a daily as well as a yearly motion. 

787. The axis of a planet is an imaginary line passing 
through its centre, and about which its diurnal revolution is 
performed. The poles of the planets are the extremities of 
this axis. 

788. The orbits of Mercury and Venus are within that 
of the Earth, and consequently they are called inferior plan¬ 
ets. The orbits of all the other planets are without, or ex¬ 
terior to that of the Earth, and these are called superior 
planets. 

That the orbits of Mercury and Venus are within that of 
the Earth, is evident from the circumstance that they are 
never seen in opposition to the Sun, that is, they never ap¬ 
pear in the west when the Sun is in the east. On the con¬ 
trary, the orbits of all the other planets are proved to be out- 


In what direction do the planets move around the 1 un ? What is the orbit 
of a planet ? What revolutions have the planets, besides their yearly revolu¬ 
tions ? Have all the planets diurnal revolutions ? How is it known that the 
planets have daily revolutions ? What is the axis of a planet? What is the 
pole of a planet? Which are the superior, and which the inferior planets? 
flow is it proved that the inferior planets are within the Earth’s orbit, and the 

superior ones without it ? f ’ 




ASTRONOMY. 243 

side of the Earth’s, since these planets are sometimes seen in 
opposition to the Sun. 

This will be understood by fig. 199, where suppose s to 
be the Sun, m the orbit of Mercury or Venus, e the orbit of 
the Earth, and j that of Jupiter. Now, it is evident, that if 
a spectator be placed any 
where in the Earth's or¬ 
bit, as at e, he may some¬ 
times see Jupiter in op¬ 
position to the Sun, as at 
j, because then the spec¬ 
tator would be between 
Jupiter and the Sun. But 
the orbit of Venus, being 
surrounded by that of the 
Earth, she never can come 
in opposition to the Sun, 
or in that part of the heav¬ 
ens opposite to him, as seen 
by us, because our Earth 
never passes between her 
and the Sun. 

789. Orbits Elliptical .—It has already been stated, that the 
orbits of the planets are elliptical, (754,) and that, conse¬ 
quently, these bodies are sometimes nearer the Sun than at 
others. An ellipse, or oval, has two foci, and the Sun, in¬ 
stead of being in the common centre, is always in the lower 
foci of their orbits. 

The orbit of a planet 
is represented by fig. 

200, where a , d , &, e , is 
an ellipse, with its two 
foci, s and o , the Sun 
being in the focus s , 
which is called the 
lower focus. 

When the Earth, or 
any other planet, re¬ 
volving around the Sun, 
is in that part of its orbit nearest the Sun, as at a, it is said 
to be in its perihelion; and when in that part^hich is at the 
greatest distance from the Sun, as at b , it is said to be in its 


Fig. 200. 

d 



Fig. 199. 



Explain fig. 199, and show why the inferior planets never can be in opposi 
tion to the Sun. What are the shapes of the planetary orbits? 






244 


ASTRONOMY. 


aphelion. The line s , d , is the mean, or average distance of 
a planet’s orbit from the Sun. 

790. Ecliptic. —The planes of the orbits of all the planets 
pass through the centre of the Sun. The plane of an orbit 
is an imaginary surface, passing from one extremity, or side 
of the orbit, to the other. If the rim of a drum head be con¬ 
sidered the orbit, its plane would be the parchment extended 
across it, on which the drum is beaten. 

Let us suppose the Earth’s orbit to be such a plane, cut¬ 
ting the Sun through his centre, and extending out on every 
side to the starry heavens ; the great circle so made, would 
mark the line of the ecliptic , or the Sun’s apparent path 
through the heavens. 

This circle is called the Sun’s apparent path, because the 
revolution of the Earth gives the Sun the appearance of pass¬ 
ing through it. It is called the ecliptic, because eclipses 
happen when the Moon is in, or near, this apparent path. 

791. Zodiac.— The Zodiac is an imaginary belt , or broad 
circle , extending quite around the heavens. The ecliptic di¬ 
vides the zodiac into two equal parts, the zodiac extending 
8 degrees on each side of the ecliptic, and therefore is 16 de¬ 
grees wide. The zodiac is divided into 12 equal parts, called 
the signs of the zodiac. 

792. The sun appears every year to pass around the great 
circle of the ecliptic, and consequently, through the 12 con¬ 
stellations, or signs of the zodiac. But it will be seen, in 
another place, that the Sun, in respect to the Earth, stands 
still, and that his apparent yearly course through the heavens 
is caused by the annual revolution of the Earth around its orbit. 

To understand the cause of 
this deception, let us suppose 
that Sj fig. 201, is the Sun, a 6, 
a part of the circle of the eclip¬ 
tic, and c d, a part of the Earth’s 
orbit. Now if a spectator be 
placed at c, he will see the Sun 
in that part of the ecliptic mark¬ 
ed by £>, but when the Earth 
moves in her annual revolution 
to d , the spectator will see the 
Sun in that part of the heavens 
marked by a; so that the mo¬ 
tion of the Earth in one direction, 
will give the Sun an apparent 
motion in the contrary direction. 


Fig. 201. 



ASTRONOMY. 


2 *5 


793. Constellations .—A sign or constellation , is a col 
lection of fixed stars, and as we have already seen, the Sur 
appears to move through the twelve signs of the zodiac every 
year. Now, the Sun’s place in the heavens, or zodiac, is 
found by his apparent conjunction, or nearness to any par¬ 
ticular star in the constellation. Suppose a spectator at c, 
observes the Sun to be nearly in a line with the star at b , then 
the Sun would be near a particular star in a certain constel¬ 
lation. When the Earth moves to cZ, the Sun’s place would 
assume another direction, and he would seem to have moved 
into another constellation, and near the star a. 

794. Each of the 12 signs of the zodiac is divided into 
30 smaller parts, called degrees; each degree into 60 equal 
parts, called minutes, and each minute into 60 parts, called 
seconds. 

The division of the zodiac into signs, is of very ancient 
date, each sign having also received the name of some ani¬ 
mal, or thing, which the constellation, forming that sign, 
was supposed to resemble. It is hardly necessary to say, 
that this is chiefly the result of imagination, since the figures 
made by the places of the stars, never mark the outlines of 
the figures of animals, or other things. This is, however, 
found to be the most convenient method of finding any par¬ 
ticular star at this day, for among astronomers, any star, in 
each constellation, may be designated by describing the part 
of the animal in which it is situated. Thus, by knowing 
how many stars belong to the constellation Leo, or the Lion, 
we readily know what star is meant by that which is situa¬ 
ted on the Lion’s ear or tail. 

795. Names of the Signs .—The names of the 12 signs of 
the zodiac are, Aries, Taurus, Gemini, Cancer, Leo, Virgo, 
Libra, Scorpia, Sagittarius, Capricorn, Aquarius, and Pisces. 
The common names, or meaning of these words, in the same 
order, are, the Ram, the Bull, the Twins, the Crab, the Lion, 
the Virgin, the Scales, the Scorpion, the Archer, the Goat, 
the Waterer, and the Fishes. 


What is meant by perihelion ? What is the plane of an orbit ? Explain 
what is meant by the ecliptic. Why is the ecliptic called the Sun’s apparent 
path? What is the zodiac ? How does the ecliptic divide the zodiac? How 
far does the zodiac extend on each side of the ecliptic? Explain fig. 201, and 
show why the Sun seems to pass through the ecliptic, when the Earth only re¬ 
volves around the Sun ? What is a constellation, or sign ? How is the Sun’s 
apparent place in the heavens found? Into how many parts are the signs of 
the zodiac divided, and what are these parts called ? Is there any resemblance 
between the places of the stars, and the figures of the animals after which 
they are called? Explain why this is a convenient method of finding any 
particular star in a sign. What are the names of the twelve signs ? 

21* 



ASTRONOMY. 


240 

The twelve signs of the zodiac, together with the Sun, 
and the Earth revolving around him, are represented at fig. 
202. When the Earth is at A, the Sun will appear to be 


Fig. 202. 



just entering the sign Aries, because then, when seen from 
the Earth, he ranges towards certain stars at the beginning of 
that constellation. When the Earth is at C, the Sun will 
appear in the opposite part of the 'heavens, and therefore in 
the beginning of Libra. The middle line, dividing the cir¬ 
cle of the zodiac into equal parts, is the line of the ecliptic. 

796. Density of the Planets. —Astronomers have no , 
means of ascertaining whether the planets are composed of 
the same kind of matter as our Earth, or whether their sur¬ 
faces are clothed with vegetables and forests, or not. They 
have, however, been able to ascertain the densities of several 


Explain why the Sun will be in the beginning of Aries, when the Earth is 
at A, fig. 202. How has the density of the planets been ascertained ? 
















I 


A.STRONOMV. 


247 


of them, by observations on their mutual attraction. By 
density , is meant compactness, or the quantity of matter in a 
given space. When two bodies are of equal bulk, that 
which weighs most, has the greatest density. It was shown, 
while treating of the properties of bodies , that substances 
attract each other in proportion to the quantities of matter 
they contain. If, therefore, we know the dimensions of 
several bodies, and can ascertain the proportion in which they 
attract each other, their quantities of matter, or densities, are 
easily found. 

797. Thus, when the planets pass each other in their cir¬ 
cuits through the heavens, they are often drawn a little out 
of the lines of their orbits by mutual attraction. As bodies 
attract in proportion to their quantities of matter, it is obvi¬ 
ous that the small planets, if of the same density, will suffer 
greater disturbance from this cause, than the large ones. 
But suppose two planets, of the same dimensions, pass each 
other, and it is found that one of them is attracted twice as 
far out of its orbit as the other, then, by the known laws of 
gravity, it would be inferred, that one of them contained twice 
the quantity of matter that the other did, and therefore that 
the density of the one was twice that of the other. 

By calculations of this kind, it has been found, that the 
density of the Sun is but a little greater than that of water, 
while Mercury is more than nine times as dense as water, 
having a specific gravity nearly equal to that of lead. The 
Earth has a density about five times greater than that of the 
Sun, and a little less than half that of Mercury. The den¬ 
sities of the other planets seem to diminish in proportion as 
their distances from the Sun increase, the density of Saturn, 
one of the most remote of planets, being only about one-third 
that of water. 

THE SUN. 


798. The Sun is the centre of the solar system , and the great 
dispenser of heat and light to all the planets. Around the Sun 
all the planets revolve , as around a common centre , he being the 
largest body in our system , and, so far as we know , the largest 
in the universe . 

799. Distance of the Sun. —The distance of the Sun from 


What is meant by density ? In what proportion do bodies attract each other ? 
How are the densities of the planets ascertained ? What is the density of the 
Sun, of Mercury, and of the Earth? In what proportions do the densities of 
the planets appear to diminish ? Where is the place of the Sun, in the solar 
<ystem ? What is the distance of the Sun from the Earth ? 



ASTRONOMY. 


248 

the Earth is 95 millions of miles, and his diameter is esti 
mated at 880,000 miles. Our globe when compared with 
the magnitude of the Sun, is a mere point, for his bulk is 
about thirteen hundred thousand times greater than that of 
the Earth. Were the Sun’s centre placed in the centre of 
the Moon’s oribt, his circumference would reach two hundred 
thousand miles beyond her orbit in every direction, thus fill¬ 
ing the whole space between us and the Moon, and extend¬ 
ing nearly as far beyond her as she is from us. A traveller, 
who should go at the rate of 90 miles a day, would perform 
a journey of nearly 33,000 miles in a year, and yet it would 
take such a traveller more than 80 years to go round the cir¬ 
cumference of the Sun. A body of such mighty dimensions, 
hanging on nothing, it is certain, must have emanated from 
an Almighty power. 

800. The Sun appears to move around the Earth every 24 
hours, rising in the east, and setting in the west. This mo¬ 
tion, as will be proved in another place, is only apparent, and 
arises from the diurnal revolution of the Earth. 

801. Diurnal revolution of the Sun .—The Sun, although 
he does not, like the planets, revolve in an orbit, is, however, 
not without motion, having a revolution around his own axis, 
once in 25 days and 10 hours. Both the fact that he has 
such a motion, and the time in which it is performed, have 
been ascertained by the spots on his surface. If a spot is 
seen, on a revolving body, in a certain direction, it is obvious, 
that when the same spot is again seen, in the same direction, 
that the body has made one revolution. By such spots the 
diurnal revolutions of the planets, as well as the Sun, have 
been determined. 

802. Spots on the Sun .—Spots on the Sun, seem first to 
have been observed in the year 1611, since which time they 
have constantly attracted attention, and have been the sub¬ 
ject of investigation among astronomers. These spots 
change their appearance as the Sun revolves on his axis, and 
become greater or less, to an observer on the Earth, as they 
are turned to, or from him ; they also change in respect to 
real magnitude and number: one spot, seen by Dr. Herschel,' 
wns estimated to be more than six times the size of our 


What is the diameter of the Sun ? Suppose the centre of the Sun and that 
of the Moon’s orbit to be coincident, how far would the Sun extend beyond the 
Moon’s orbit ? How is it proved that the Sun has a motion around his own 
axis ? How often does the Sun revolve ? When were spots on the Sun first 
observed ? 



ASTRONOMY. 


249 

Earth, being 50,000 miles in diameter. Sometimes forty or 
fifty spots may be seen at the same time, and sometimes only 
one. They are often so large as to be seen with the naked 
eye ; this was the case in 1816. 

803. Nature and design of these Spots .—In respect to the 
nature and design of these spots, almost every astronomer has 
formed a different theory. Some have supposed them to be 
solid opaque masses of scoriae, floating in the liquid fire of the 
Sun ; others, as satellites, revolving round him, and hiding his 
light from us ; others, as immense masses, which have fallen 
on his disc, and which are dark colored, because they have 
not yet become sufficiently heated. In two instances, these 
spots have been seen to burst into several parts, and the parts 
to fly in several directions, like a piece of ice thrown upon 
the ground. Others have supposed that these dark spots 
were the body of .the Sun, which became visible in conse¬ 
quence of openings through the fiery matter, with which he 
is surrounded. Dr. Herschel, from many observations with 
his great telescope, concludes, that the shining matter of the 
Sun consists of a mass of phosphoric clouds, and that the 
spots on his surface are owing to disturbances in the equili¬ 
brium of this luminous matter, by which openings are made 
through it. There are, however, objections to this theory, 
as indeed there are to all the others, and at present it can 
only be said, that no satisfactory explanation of the cause of 
these spots has been given. 

804. The Sun inhabited .—That the Sun, at the same time 
that he is the great source of heat and fight to all the solar 
worlds, may yet be capable of supporting animal fife, has 
been the favorite doctrine of several able astronomers. Dr. 
Wilson first suggested that this might be the case, and Dr. 
Herschel, with his telescope, made observations which con¬ 
firmed him in this opinion. The latter astronomer supposed 
that the functions of the Sun as the dispenser of light and 
heat, might be performed by a luminous, or phosphoric at¬ 
mosphere, surrounding him at many hundred miles distance, 
while his solid nucleus might be fitted for the habitations of 
millions of reasonable beings. This doctrine is, however, 
rejected by most writers on the subject at the present day. 


What has been the difference in the number of spots observed ? What was 
the size of the spots seen by Dr. Herschel? What has been advanced con¬ 
cerning the nature of these spots ? Have they been accounted for satisfac¬ 
torily ? What is said concerning the Sun’s being a habitable globe ? 




ASTRONOMY 


250 


MERCURY. 

805. Mercury , the planet nearest the sun, is about 3,000 
miles in diameter, and revolves around him, at the distance 
of 37 millions of miles. The period of his annual revolu¬ 
tion is 87 days, and he turns on his axis once in about 24 
hours. 

The nearness of this planet to the Sun, and the short time 
his fully illuminated disc is turned towards the earth, has 
prevented astronomers from making many observations on 
him. 

No signs of an atmosphere have been observed in this 
planet. The Sun’s heat at Mercury is about seven times 
greater than it is on the Earth, so that water, if nature fol¬ 
lows the same laws there that she does here, cannot exist at 
Mercury, except in the state of steam. 

The nearness of this planet to the Sun, prevents his being 
often seen. He may, however, sometimes be observed just 
before the rising, and a little after the setting of the Sun. 
When seen after sunset, he appears a brilliant, twinkling star, 
showing a white light, which, however, is much obscured 
by the glare of twilight. When seen in the morning, before 
the rising of the Sun, his light is also obscured by the Sun’s 
rays. 

Mercury sometimes crosses the disc of the Sun, or comes 
between the Earth and that luminary, so as to appear like a 
small dark spot passing over the Sun’s face. This is called 
the transit of Mercury. 


VENUS. 

806. Venus is the other planet, whose orbit is within that 
of the Earth. Her diameter is about 8,600 miles, being some¬ 
what larger than the Earth. 

Her revolution around the Sun is performed in 224 days, 
at the distance of 68 millions of miles from him. She turns 
on her axis once in 23 hours, so that her day is a little shorter 
than ours. 

807. Venus, as seen from the Earth, is the most brilliant 
of all the primary planets, and is better known than any 


What is the diameter of Mercury, and what are his periods of annual and 
diurnal revolution? How great is the Sun’s heat at Mercury ? At what times 
is Mercury to be seen 1 What is a transit of Mercury ? Where is the orbit of 
Venus, in respect to that of the Earth ? What is the time of Venus’ revolution 
round the Sun ? How often does she turn on her axis ? 




ASTRONOMY. 


251 


nocturnal Luminary except the Moon. When seen through 
a telescope, she exhibits the phases or horned appearance of 
the moon, and her face is sometimes variegated with dark 
spots. Venus may often be seen in the day time, even when 
she is in the vicinity of the blazing light of the Sun. A 
luminous appearance around this planet, seen at certain 
times, proves that she has an atmosphere. Some of her 
mountains are several times more elevated than any on our 
globe, being from 10 to 22 miles high. Venus sometimes 
makes a transit across the Sun’s disc, in the same manner as 
Mercury, already described. The transits of Venus occur 
only at distant periods from each other. The last transit 
was in 1769, and the next will not happen until 1874. 
These transits have been observed by astronomers with the 
greatest care and accuracy, since it is by observations on 
them that the true distances of the Earth and planets from the 
Sun are determined. 

When Venus is in that part of her orbit which gives her 
the appearance of being west of the Sun, she rises before him, 
and is then called the morning star ; and when she appears 
east of the Sun, she is behind him in her course, and is then 
called the evening star. These periods do not agree, either 
with the yearly revolution of the Earth, or of Venus, for she 
is alternately 290 days the morning star, and 290 days the 
evening star. The reason of this is, that the Earth and Ve¬ 
nus move round the Sun in the same direction, and hence 
her relative motion, in respect to the Earth, is much slower 
than her absolute motion in her orbit. If the Earth had no 
yearly motion, Venus would be the morning star one half of 
the year, and the evening star the other half. 


THE EARTH. 

809. The next planet in our system, nearest the Sun, is 
the Earth. Her diameter is 7,912 miles. This planet, re¬ 
volves around him in 365 days 5 hours, and 48 minuu s; 
and at the distance of 95 millions of miles. It turns round 
its own axis once in 24 hours, making a day and a night. 
The Earth’s revolution around the Sun is called its annual or 
yearly motion, because it is performed in a year; while the 
revolution around its own axis, is called the diurnal or daily 


What is said of the height of the mountains m Venus? On what account 
are the transits of Venus observed with great care ? When is Venus the 
morning, and when the evening star? How long is Venus the morning, and 
how long the evening star? How long does it take the Earth to revolve round 
the Sun ? 




ASTRONOMY. 


252 

motion, because it takes place every day. The figure of the«* 
Earth, with the phenomena connected with her motion, will 
be explained in another place. 

THE MOON. 

810. The Moon, next to the Sun, is, to us, the most bril 
liant and interesting of all the celestial bodies. Being the: 
nearest to us of any of the heavenly orbs, and apparently de¬ 
signed for our use, she has been observed with great atten¬ 
tion, and many of the phenomena which she presents, are 
therefore better understood and explained, than those of the 
other planets. 

While the Earth revolves round the Sun in a year, it is at¬ 
tended by the Moon, which makes a revolution round the 
Earth once in 27 days, 7 hours, and 43 minutes. The dis¬ 
tance of the Moon from the Earth is 240,000 miles, and her 
diameter about 2,000 miles. 

Her surface, when seen through a telescope, appears diver¬ 
sified with hills, mountains, valleys, rocks, and plains, present¬ 
ing a most interesting and curious aspect: but the explana¬ 
tion of these phenomena are reserved for another section. 

MARS. 

811. The next planet in the solar system, is Mars, his or¬ 
bit surrounding that of the Earth. The diameter of this 
planet is upwards of 4,000 miles, being about half that of the 
Earth. The revolution of Mars around the Sun is performed 
in nearly 687 days, or in somewhat less than two of our 
years, and he turns on his axis once in 24 hours and 40 min¬ 
utes. His mean distance from the Sun is 144 millions of 
miles, so that he moves in his orbit at the rate of about 
55,000 miles in an hour. The days and nights, at this 
planet, and the different seasons of the year, bear a considera¬ 
ble resemblance to those of the Earth. The density of Mars 
is less than that of the Earth, being only three times that 
of water. 

Mars reflects a dull red light, by which he may be distin- - 
guished from the other planets. His appearance through th<! 


What is meant by the Earth’s annual revolution, and what by her diurnal 
revolution ? Why are the phenomena of the Moon better explained than those 
of the other planets ? In what time is a revolution of the Moon about the Earth 

S erformed ? What is the distance of the Moon from the Earth ? What is the 
iameter of Mars ? How much longer is a year at Mars than our year ? What 
is his rate of motion in his orbit ? What is his appearance through the tele 
scope? 




ASTRONOMY. 


253 


telescope is remarkable for the great number and variety of 
spots which his surface presents. 

Mars has an atmosphere of great density and extent, as is 
proved by the dim appearance of the fixed stars, when seen 
through it. When any of the stars are seen nearly in a line 
with this planet, they give a faint, obscure light, and the 
nearer they approach the line of his disc, the fainter is their 
light, until the star is entirely obscured from the sight. 

This planet sometimes appears much larger to us than at 
others, and this is readily accounted for by his greater or 
less distance. At his nearest approach to the Earth, his 
distance is only 50 millions of miles, while his greatest dis¬ 
tance is 240 millions of miles; making a difference in his 
distance of 190 millions of miles, or the diameter of the 
Earth’s orbit. 

The Sun’s heat at this planet is less than half that which 
we enjoy. 

To the inhabitants of Mars, our planet appears alternately 
as the morning and evening star, as Venus does to us. 

VESTA, JUNO, PALLAS, AND CERES. 

812. These planets were unknown until recently, and are 
therefore sometimes called the new planets. It has been 
mentioned, that they are also called Asteroids. 

813. The orbit of Vesta is next in the solar system to that 
of Mars. This planet was discovered by Dr. Olbers, of 
Bremen, in 1807. .The light of Vesta is of a pure white, and 
in a clear night she may be seen with the naked eye, appear¬ 
ing about the size of a star of the 5th or 6th magnitude. 
Her revolution round the Sun is performed in three years and 
66 days, at the distance of 223 millions of miles from him. 

814. Juno was discovered by Mr. Harding, of Bremen, in 
1804. Her mean distance from the Sun is 253 millions of 
miles. Her orbit is more elliptical than that of any other 
planet, and, in consequence, she is sometimes 127 millions of 
miles nearer the Sun than at others. This planet completes 
its annual revolution in 4 years and about 4 months, and 
revolves round its axis in 27 hours. Its diameter is 1400 
miles. 

How is it proved that Mars has an atmosphere of great density? Why does 
Mars sometimes appear to us larger than at others ? How great is the Sun’s 
heat at Mars? Which are the new planets, or asteroids? When was Vesta 
discovered? What is the period of Vesta’s annual revolution? When was 
Juno discovered ? What is her distance from the Sun ? What is the period 
of her revolution, and what her diameter ? 

22 





254 


ASTRONOMY. 


815. Pallas was also discovered by Dr. Olbers, in 1802. 
It* distance from the Sun is 226 millions of miles, and its 
periodic revolution round him, is performed in 4 years and 
7 months. 

816. Ceres was discovered in 1801, by Piazzi, of Palermo. 
This planet performs her revolution in the same time as Pal¬ 
las, being 4 years and seven months. Her distance from the 
Sun, 260 millions of miles. According to Dr. Herschel, this 
planet is only about 160 miles in diameter. 

JUPITER. 

817. Jupiter is 89,000 miles in diameter , and performs his 
annual revolution once in about 11 years , at the distance of 
490 million>s of miles from the Sun. This is the largest planet 
in the solar system, being about 1,400 times larger than the 
Earth. His diurnal revolution is performed in nine hours and 
fifty-five minutes, giving his surface, at the equator, a motion 
of 28,000 miles per hour. This motion is about twenty 
times more rapid than that of our Earth at the equator. 

818. Jupiter, next to Venus, is the most brilliant of the 
planets, though the light and heat of the Sun on him is 
nearly 25 times less than on the Earth. 

This planet is distinguished from all the others, by an 
appearance resembling bands, which extend across^his disc. 

Fig. 203. 



These are termed belts , and are variable, both in respect to 
number and appearance. Sometimes seven or eight are seen, 


What is said of Pallas and Ceres? What is the diameter of Jupiter» 
What is his distance from the Sun? What is the period of Jupiter’s diurnal 
revolution ? What is the Sun’s heat and light at Jupiter, when compared with 
that of the Earth? For what is Jupiter particularly distinguished? Is the 
appearance of Jupiter’s belts always the same, or do they change ? 

















ASTRONOMV. 


255 

several of which extend quite across his face, while others 
appear broken, or interrupted. 

These bands, or belts, when the planet is observed through 
a telescope, appear as represented in fig. 203. This appear¬ 
ance is much the most common, the belts running quite 
across the face of the planet in parallel lines. Sometimes, 
however, his aspect is quite different from this, for in 1780, 
Dr. Herschel saw the whole disc of Jupiter covered with 
small curved lines, each of which appeared broken, or inter¬ 
rupted, the whole having a parallel direction across his disc, 
as in fig. 204. 


Fig. 204. 



Different opinions have been advanced by astronomers 
respecting the cause of these appearances. By some they 
have been regarded as clouds, or as openings in the atmos¬ 
phere of the planet, while others imagine that they are the 
marks of great natural changes, or revolutions, which are 
perpetually agitating the surface of that planet. It is, how¬ 
ever, most probable, that these appearances are produced by 
the agency of some cause, of which we, on this little Earth, 
must always be entirely ignorant. 

819. Jupiter has four satellites, or moons, two of which 
are sometimes seen with the naked eye. They move round, 
and attend him in his yearly revolution, as the Moon does 
our Earth. They complete their revolutions at different 
periods, the shortest of which is less than two days, and the 
longest seventeen days. 

Eclipses of Jupiter's Moons .—These satellites often fall 
into the shadow of their primary, in consequence of which 
they are eclipsed, as seen from the Earth. The eclipses of 

What is said of the cause of Jupiter’s belted appearance ? How many 
moons has Jupiter, and what are the periods of their revolutions ? What 
occasions the eclipses of Jupiter’s moons ? 
















256 


ASTRONOMY. 


Jupiter’s moons have been observed with great care by 
astronomers, because they have been the means of determin¬ 
ing the exact longitude of places, and the velocity with 
which light moves through space. How longitude is deter 
mined by these eclipses, cannot be explained or understood 
at this place, but the method by which they become the 
means of ascertaining the velocity of light, may be readily 
comprehended. An eclipse of one of these satellites appears, 
by calculation, to take place sixteen minutes sooner, when 
the Earth is in that part of her orbit nearest to Jupiter, than 
it does when the Earth is in that part of her orbit at the 
greatest distance from him. Hence, light is found to be six¬ 
teen minutes in crossing the Earth’s orbit, and as the Sun is 
in the centre of this orbit, or nearly so, it must take about 8 
minutes for the light to come from him to us. Light, there¬ 
fore, passes at the velocity of 95 millions of miles, our dis¬ 
tance from the Sun, in about 8 minutes, which is nearly 200 
thousand miles in a second. 

SATURN. 

820. The planet Saturn revolves round the Sun in a period 
of about 30 o f our years , and at the distance from him of 900 
millions of miles. His diameter is 79,000 miles, making his 
bulk nearly nine hundred times greater than that of the 
Earth, but notwithstanding this vast size, he revolves on his 
axis once in about ten hours. Saturn, therefore, performs 
upwards, of 25,000 diurnal revolutions in one of his years, 
and hence his year consists of more than 25,000 days; a 
period of time equal to more than 10,000 of our days. On 
account of the remote distance of Saturn from the Sun, he 
receives only about a 90th part of the heat and light which 
we enjoy on the Earth. But to compensate, in some degree, 
for this vast distance from the Sun, Saturn has seven moons, 
which revolve round him at different distances, and at various 
periods, from 1 to 80 days. 

821. Rings of Saturn. —Saturn is distinguished from the 
other planets by his ring , as Jupiter is by his belt. When 
this planet is viewed through a telescope, he appears sur- 


Of what use are these eclipses to astronomers ? How is the velocity of 
light ascertained by the eclipses of Jupiter’s satellites? What is the time of 
Saturn’s periodic revolution round the Sun ? What is his distance from the 
Sun? What his diameter? What is the period of his diurnal revolution? 
How many days make a year at Saturn? How many moons has Saturn? 
How is Saturn particularly distinguished from all the other planets? 



ASTRONOMY. 


257 

rounded by an immense luminous circle, which is represented 
by fig. 205. 

There are indeed two luminous circles, or rings, one with¬ 
in the other, with a dark space between them, so that they 
do not appear to touch each other. Neither does the inner 
ring touch the 

body of the Fi s- 205 - 

planet, there 
being by esti¬ 
mation, about 
the distance 
of thirty thou¬ 
sand miles be¬ 
tween them. 

The exter¬ 
nal circum¬ 
ference of the outer ring is 640,000 miles, and its breadth 
from -the outer to the inner circumference, 7,200 miles, or 
nearly the diameter of our Earth. The dark space, between 
the two rings, or the interval between the inner and the outer 
ring, is 2,800 miles. 

This immense appendage revolves round the Sun with 
the planet,—performs daily revolutions with it. and, accord¬ 
ing to Dr. Herschel, is a solid substance, equal in density to 
the body of the planet itself. 

822. The design of Saturn’s ring, an appendage so vast, 
and so different from any thing presented by the other plan¬ 
ets, has always been a matter of speculation and inquiry 
among astronomers. One of its most obvious uses appears 
to be that of reflecting the light of the Sun on the body of 
the planet, and possibly it may reflect the heat also, so as in 
some degree to soften the rigor of so inhospitable a climate. 

823. As this planet revolves around the Sun, one of its 
sides is illuminated during one half of the year, and the other 
side during the other half; so that, as Saturn’s year is equal 
to thirty of our years, one of his sides will be enlightened and 
darkened, alternately, every fifteen years, as the poles of our 
Earth are alternately in the light and dark every year. 

Fig. 206 represents Saturn as seen by an eye, placed -at 
right-angles to the plane of his ring. When seen from the 
Earth, his position is always oblique as represented by fig. 205. 

What distance is there between the body of Saturn and his inner ring ? 
What distance is there between his inner and outer ring ? What is the cir¬ 
cumference of the outer ring ? How long is one of Saturn’s sides alternately 
in the light and dark ? 

22 * 







258 


astronomy. 


The inner white cir¬ 
cle represents the body 
of the planet, enlight¬ 
ened by the Sun. The 
dark circle next to this, 
is the unenlightened 
space between thebody 
of the planet and the 
inner ring, being the 
dark expanse of the 
heavens beyond the 
planet. The two white 
circles are the rings of 
the planet, with the 
dark space between 
them, which also is the 
dark expanse of the heavens. 

HERSCHEL. 

824. In consequence of some inequalities in the motions 
of Jupiter and Saturn, in their orbits, several astronomers 
had suspected that there existed another planet beyond the 
orbit of Saturn, by whose attractive influence these irregu¬ 
larities were produced. This conjecture was confirmed by 
Dr. Herschel, in 1781, who in that year discovered the planet, 
which is now generally known by the name of its discov¬ 
erer, though called by him Georgium sidus. The orbit of 
Herschel is beyond that of Saturn, and at the distance of 
1800 millions of miles from the Sun. To the naked eye this 
planet appears like a star of the sixth magnitude, being, with 
the exception of some of the comets, the most remote body, 
so far as is known, in the solar system. 

825. Herschel completes his revolution round the Sun in 
nearly 84 of our years, moving in his orbit at the rate of 
15,000 miles in an hour. His diameter is 35,000 miles, so that 
his bulk is about eighty times that of the Earth. The light 
and heat of the Sun at Herschel, is about 360 times less 
than it is at the Earth, and yet it has been found, by calcu¬ 
lation. that this light is equal to 248 of our full Moons, a 


Iii what, position is Saturn represented by fig. 206 ? What circumstance 
led to the discovery of Herschel ? In what year, and by whom, was Herschel 
discovered? What is the distance of Herschel from the Sun? In what peri¬ 
od is his revolution round the Sun performed ? What is the diameter of Her¬ 
schel ? What is the quantity of light and heat at Herschel, whem compared 
with that of the Earth r 




ASTRONOMY. 


259 



Herschel 


826. Relative Situations of the Planets. —Having 
now given a short account of each planet composing the so¬ 
lar system, the relative situation of their several orbits, with 
the exception of those of the Asteroids, are shown by fig. 207. 

In this figure, the orbits are marked by the signs of each 
planet, of which the first, or that nearest the Sun, is Mer¬ 
cury, the next Venus, the third the Earth, the fourth Mars ; 
then come those of the Asteroids, then Jupiter, then Saturn, 
and lastly Herschel. 

827. Comparative Dimensions of the Planets. —The 
comparative dimensions of the planets are delineated at fig. 
208 . 


striking proof of the inconceivable quantity of light emitted 
by the Sun. 

This planet has six satellites, which revolve round him at 
various distances, and in different times. The periods of some 
of these have been ascertained, while those of the others re¬ 
main unknown. 


Fig. 207. 







260 


ASTRONOMY. 


Fig. 208. 



MOTIONS OF THE PLANETS. 

828. It is said, that when Sir Isaac Newton was near de¬ 
monstrating the great truth, that gravity is the cause which 
keeps the heavenly bodies in their orbits, he became so agi 
tated with the thoughts of the magnitude and consequences 
of his discovery, as to be unable to proceed with his demon¬ 
strations, and desired a friend to finish what the intensity 
of his feelings would not allow him to complete. 

We have seen, in a former part of this work, (150) that 
all undisturbed motion is straight forward, and that a body 
projected into open space, would continue, perpetually, to 
move in a right line, unless retarded or drawn out of this 
course by some external cause. 

829. To account for the motions of the planets in their 
orbits, we will suppose that the Earth, at the time of its cre¬ 
ation, was thrown by the hand of the Creator into open 
space, the Sun having been before created and fixed in his 
present place. 

830. Circular Motion of the Planets .—Under Compound 
motion , (160,) it has been shown, that when a body is acted 
on by two forces perpendicular to each other, its motion will 
be in a diagonal between the direction of the two forces. 

But we will again here 
suppose that a ball is mov¬ 
ing in the line m «, fig. 

209, with a given force, oc 
and that another force half 
as great should strike it in 
the direction of n, the ball 
would then describe the o 








ASTRONOMY. 


261 


diagonal of a parallelogram, whose length would be just 
equal to twice its breadth, and the line of the ball would be 
straight, because it would obey the impulse and direction of 
these two forces only. 

Now let «, fig. 210, Fig- 210 . 

represent the Earth, 
and S the Sun ; and 
suppose the Earth to 
be moving forward, in 
the line from a to 6, 
and to have arrived at 
a, with a velocity suf¬ 
ficient, in a given time, 
and without distur¬ 
bance, to have carried 
it to b. But at the 
point 0 , the Sun, S, 
acts upon the Earth 
with his attractive power, and with a force which would 
draw it to c, in the same space of time that it would other¬ 
wise have gone to b. Then the Earth, instead of passing 
to b , in a straight line, would be drawn down to e?, the diag¬ 
onal of the parallelogram a , b. d , c. The line of direction, 
in fig. 209, is straight, because the body moved obeys only 
the direction of the two forces, but it is curved from a to d , 
fig. 210, in consequence of the continued force of the Sun’s 
attraction, which produces a constant deviation from a right 
line. 

When the Earth arrives at d, still retaining its projectile 
or centrifugal force, its line of direction would be towards rc, 
but while it would pass along to n without disturbance, the at¬ 
tracting force of the Sun is again sufficient to bring it to e, 
in a straight line, so that, in obedience to the two impulses, 
it again describes the curve to o. 

831. It must be remembered, in order to account for the 
circular motions of the planets, that the attractive force of 
the Sun is not exerted at once, or by a single impulse, as is 
the case with the cross forces, producing a straight line, but 
that this force is imparted by degrees, and is constant. It 
therefore acts equally on the Earth, in all parts of the course 

Suppose a body to be acted on by two forces perpendicular to each other, in 
what direction will it move ? Why does the ball, fig. 209, move in a straight 
line ? Why does the Earth, fig. 210, move in a curved line ? Explain fig, 210, 
and show how the two forces act to produce a circular line of motion ? What 
is the projectile force of the Earth called ? 










ASTRONOMY. 


262 

from a to d, and from d to o. From o , the Earth having the 
same impulses as before, it moves in the same curved or cir¬ 
cular direction, and thus its motion is continued perpetually. 

832. The tendency of the Earth to move forward in a 
straight line, is called the centrifugal force , and the attraction 
of the Sun, by which it is drawn downwards, or towards a 
centre, is called its centripetal force, and it is by these two 
forces that the planets are made to perform their constant 
revolutions around the Sun. 

833. Elliptical Orbits. 

—In the above explana¬ 
tion, it has been sup¬ 
posed that the Sun’s at¬ 
traction, which consti¬ 
tutes the Earth’s grav¬ 
ity, was at all times 
equal, or that the Earth 
was at an equal dis¬ 
tance from the Sun, in 
all parts of its orbit. 

But, as heretofore ex¬ 
plained, the orbits of all 
the planets are elliptic¬ 
al, the Sun being placed 
in the lower focus of 
the ellipse. The Sun’s 
attraction is, therefore, stronger in some parts of their orbits 
than in others, and for this reason their velocities are greater 
at some periods of their revolutions than at others. 

To make this understood, suppose, as before, that the cen¬ 
trifugal and centripetal forces so balance each other, that the 
Earth moves round the circular orbit a e b, fig. 211, until it 
comes to the point e; and at this point, let us suppose, that 
the gravitating force is too strong for the force of projection, 
so that the Earth, instead of continuing its former direction 
towards b, is attracted by the Sun s , in the curve e c. When 
at c, the line of the Earth’s projectile force, instead of tending 
to carry it farther from the Sun, as would be the case were 
it revolving in a circular orbit, now tends to draw it still nearer 
to him, so that at this point, it is impelled by both forces to¬ 
wards the Sun. From c, therefore, the force of gravity in- 


Fig. 211. 


e 



What is the attractive force of the Sun, which draws the Earth towards him, 
called? Explain fig. 211, and show the reason why the velocity is increased 
from c to d, and why is it not retarded from d tog? 






ASTRONOMY. 


263 


creasing in proportion as the square of the distance between 
the Sun and Earth diminishes, the velocity of the Earth will 
he uniformly accelerated, until it arrives at the point nearest 
the Sun d. At this part of its orbit, the Earth will have 
gamed, by its increased velocity, so much centrifugal force, 
as to give it a tendency to overcome the Sun’s attraction, and 
to fly off in the line d o. But the Sun’s attraction being also 
increased by the near approach of the Earth, the Earth is 
retained in its orbit, notwithstanding its increased centrifugal 
force, and it therefore passes through the opposite part of its 
orbit, from d to g, at the same distance from him that it ap¬ 
proached. As the Earth passes from the Sun, the force of 
gravity tends continually to retard its motion, as it did to in¬ 
crease it while approaching him. But the velocity it had 
acquired in approaching the Sun, gives it the same rate of 
motion from d to g, that it had from c to d. From g , the 
Earth’s motion is uniformly retarded, until it again arrives at 
e, the point from which it commenced, and from whence it 
describes the same orbit, by virtue of the same forces as 
before. 

The Earth, therefore, in its journey round the Sun, moves 
at very unequal velocities, sometimes being retarded, and 
then again accelerated, by the Sun’s attraction. 

834. Planets pass equal Areas in equal times .—It is an 
interesting circumstance, 
respecting the motions of 
the planets, that if the 
contents of their orbits be 
divided into unequal tri¬ 
angles, the acute angles 
of which centre at the Sun, 
with the line of the orbit 
for their bases, the centre 
of the planet will pass 
through each of these 
bases in equal times. 

This will be understood 
by fig. 212, the elliptical 
circle being supposed to 
be the Earth’s orbit, with 
the Sun, s , in one of the 
foci. 

Now the spaces, 1,2, 3, 


Fig. 212. 



What is meant by a planet’s passing through equal spaoes in equal times ? 










264 


ASTRONOMY. 


&c. though of different shapes, are of the same dimensions, 
or contain the same quantity of surface. The Earth, we 
have already seen, in its journey round the Sun, describes an 
ellipse, and moves more rapidly in one part of its orbit than 
in another. But whatever may be its actual velocity, its 
comparative motion is through equal areas in equal times. 
Thus its centre passes from E to C, and from C to A, in the 
same period of time, and so of all the other divisions marked 
in the figure. If the figure, therefore, be considered the plane 
of the Earth’s orbit, divided into 12 equal areas, answering to 
the 12 months of the year, the Earth will pass through the 
same areas in every month, but the spaces through which it 
passes will be increased, during every month, for one half 
the year, and diminished, during every month, for the other half. 

835. Why the Planets do not fall to the Sun .—The reason 
why the planets, when they approach near the Sun, do not 
fall to him, in consequence of his increased attraction, and 
why they do not fly off into open space, when they recede 
to the greatest distance from him, may be thus explained. 

836. Taking the Earth as an example, we have shown 
that when in the part of her orbit nearest the Sun, her velo¬ 
city is greatly increased by his attraction, and that conse¬ 
quently the Earth’s centrifugal force is increased in propor¬ 
tion. As an illustration of this, we know that a thread 
which will sustain an ounce ball, when whirled round in the 
air, at the rate of 50 revolutions in a minute, would be 
broken, were these revolutions increased to the number of 
60 or 70 in a minute, and that the ball would then fly off in a 
straight line. This shows that when the motion of a revolv¬ 
ing body is increased, its centrifugal force is also increased. 
Now, the velocity of the Earth increases in an inverse pro¬ 
portion, as its distance from the Sun diminishes, and in pro¬ 
portion to the increase of velocity is its centrifugal force in¬ 
creased ; so that, in any other part of its orbit, except when 
nearest the Sun, this increase of velocity would carry the 
Earth away from its centre of attraction. But this increase 
of the Earth’s velocity is caused by its near approach to the 
Sun, and consequently the Sun’s attraction is increased, &s 
well as the Earth’s velocity. In other terms, when the cen¬ 
trifugal force is increased, the centripetal force is increased 

. How is it shown, that if the motion of a revolving body is increased, its 
projectile force is also increased? By what force is the Earth’s velocity in¬ 
creased, as it approaches the Sun ? When the Earth is nearest the Sun, why 
does it not fall to him? When the Earth’s centrifugal force is greatest, what 
prevents its flying to the Sun 1 



EARTH. 


265 

in proportion, and thus, while the centrifugal force prevents 
he haith from falling to the Sun, the centripetal force pre¬ 
vents it from moving off in a straight line. 

837. When the Earth is in that part of its orbit most dis¬ 
tant from the Sun, its projectile velocity being retarded by 
the counter force of the Sun’s attraction, becomes greatly 
diminished, and then the centripetal force becomes stronger 
than the centrifugal, and the Earth is again brought back by 
the bun’s attraction, as before, and in this manner its motion 
goes on without ceasing. It is supposed, as the planets 
move through spaces void of resistance, that their centrifugal 
forces remain the same as when they first emanated from the 
hand of the Creator, and that this force, without the influ¬ 
ence of the Sun’s attraction, would carry them forward into 
infinite space. 

THE EARTH. 

838. Proofs of the Earth's Diurnal Revolution .—It is al¬ 
most universally believed, at the present day, that the ap¬ 
parent daily motion of the heavenly bodies from east to west, 
is caused by the real motion of the Earth from west to east, 
and yet there are comparatively few who have examined the 
evidence on which this belief is founded. For this reason, 
we will here state the most obvious, and to a common ob¬ 
server, the most convincing proofs of the Earth’s revolution. 
These are, first, the inconceivable velocity of the heavenly 
bodies, and particularly the fixed stars, around the Earth, if 
she stands still. Second, the fact, that all astronomers of 
the present age agree, that every phenomenon which the 
heavens present, can be best accounted for, by supposing the 
Earth to revolve. Third, the analogy to be drawn from 
many of the other planets, which are known to revolve on 
their axes; and fourth, the different lengths of days and 
nights at the different planets, for did the Sun revolve about 
the solar system, the days and nights at many of the planets 
must be of similar lengths. 

839. The distance of the Sun from the Earth being 95 
millions of miles, the diameter of the Earth’s orbit is twice 
its distance from the Sun, and, therefore, 190 millions of 
miles. Now, the diameter of the Earth’s orbit, when seen 
from the nearest fixed star, is a mere point, and were the 


What are the most obvious and convincing proofs that the Earth revolves on 
its axis ? Were the Earth’s orbit a solid mass, could it be seen I v us, at the 
distance of the fixed stars 1 

23 



EARTH 


266 

orbit a solid mass of opaque matter, it could not be seen, 
with such eyes as ours, from such a distance. This is 
known by the fact, that these stars appear no larger to us, 
even when our sight is assisted by the best telescopes, when 
the Earth is in that part of her orbit nearest them, than when 
at the greatest distance, or in the opposite part of her orbit. 
The approach, therefore, of 190 millions of miles towards 
the fixed stars, is so small a part of their whole distance 
from us, that it makes no perceptible difference in their ap¬ 
pearance. Now, if the Earth does not turn on her axis once 
in 24 hours, these fixed stars must revolve around tne Earth 
at this amazing distance once in 24 hours. If the Sun passes 
around the Earth in 24 hours he must travel at the rate of 
nearly 400,000 miles in a minute; but the fixed stars are at 
least 400,000 times as far beyond the Sun, as the Sun is 
from us, and, therefore, if they revolve around the Earth, 
must go at the rate of 400,000 times 400,000 miles, that is, 
at the rate of 160,000,000,000, or 160 billions of miles in a 
minute; a velocity of which we can have no more concep¬ 
tion than of infinity or eternity. 

840. In respect to the analogy to be drawn from the 
known revolutions of the other planets, and the different 
length of days and nights among them, it is sufficient to 
state, that to the inhabitants of Jupiter, the heavens appear 
to make a revolution in about 10 hours, while to those of 
Venus, they appear to revolve once in 23 hours, and to the 
inhabitants of the other planets a similar difference seems 
to take place, depending on the periods of their diurnal revo¬ 
lutions. Now, there is no more reason to suppose that the 
heavens revolve round us, than there is to suppose that they 
revolve around any of the other planets, since the same ap¬ 
parent revolution is common to them all; and as we know 
that the other planets, at least many of them, turn on their 
axes, and as all the phenomena presented by the Earth, can 
be accounted for by such a revolution, it is folly to conclude 
otherwise. 


Suppose the Earth stood still, how fast must the Sun move to go round it in 
24 hours ? At what rate must the fixed stars move to go round it in 24 hours ? 
If the heavens appear to revolve every 10 hours at Jupiter, and every 24 hours 
at the Earth, how can this difference be accounted for, if they revolve at all ? 
Is there any more reason to believe that the Sun revolves round the Earth, 
than round any of the other planets ? How can all the phenomena of the 
heavens be accounted for if the planets do not revolve ? 



EARTH. 


267 


CIRCLES AND DIVISIONS OF THE EARTH. 

841. It will be necessary for the pupil to retain in his 
memory the names and directions of the following lines, or 
circles, by which the Earth is divided into parts. These 
lines, it must be understood, are entirely imaginary, there 
being no such divisions marked by nature on the Earth’s 
surface. They are, however, so necessary, that no accurate 
description of the Earth, or of its position with respect to the 
heavenly bodies, can be conveyed without them. 

The Earth, 

whose diameter is Fig. 213. 

7912 miles, is rep¬ 
resented by the 
globe, or sphere, 

% 213. The 

straight line pass¬ 
ing through its 
centre, and about 
which it turns, is 
called its axis , and 
the two extremi¬ 
ties of the axis 
are the poles of 
the Earth, A be¬ 
ing the north pole, 
and B the south 
pole,. The line C 
D, crossing the axis, passes quite round the Earth, and divides 
it into two equal parts. This is called the equinoctial line , or 
the equator. That part of the Earth situated north of this 
line, is called the northern hemisphere , and that part south of 
it, the southern hemisphere. The small circles E F and G H, 
surrounding or including the poles, are called the polar cir¬ 
cles. That surrounding the north pole is called the arctic 
circle , and that surrounding the south, the antarctic circle. 
Between these circles, there is, on each side of the equator, 
another circle, which marks the extent of the tropics towards 
the north and south, from the equator. That to the north 
of the equator, I K, is called the tropic of Cancer , and that 


What is the axis of the Earth? What are the poles of the Earth? What 
is the equator ? Where are the northern and southern hemispheres ? What 
are the polar circles ? Which is the arctic, and which the antarctic circle ? 
Where is the tropic of Cancer and where the tropic of Capricorn? 















268 


EARTH. 


to the south, L M, the tropic of Capricorn. The circle L K, 
extending obliquely across the two tropics, and crossing the 
axis of the Earth, and the equator at their point of intersec¬ 
tion, is called the ecliptic. This circle, as already explained, 
belongs rather to the heavens than the Earth, being an im¬ 
aginary extension of the plane of the Earth’s orbit in every 
direction towards the stars. The line in the figure, shows 
the comparative position or direction of the ecliptic in respect 
to the equator, and the axis of the Earth. 

The lines crossing those already described, and meeting 
at the poles of the Earth, are called meridian lines , or mid¬ 
day lines, for when the Sun is on the meridian of a place, it 
is the middle of the day at that place, and as these lines 
extend from north to south, the Sun shines on the whole 
length of each, at the same time, so that it is 12 o’clock, at 
the same time, on every place situated on the same meridian. 

The spaces on the Earth, between the lines extending from 
east to west, are called zones. That which lies between the 
tropics, from M to K, and from I to L, is called the torrid 
zone , because it comprehends the hottest portion of the 
Earth. The spaces which extend from the tropics, north 
and south, to the polar circles, are called temperate zones. 
because the climates are temperate, and neither scorched 
with heat, like the tropics, nor chilled with the cold like the 
frigid zones. That lying north of the tropic of Cancer, is 
called the north temperate zone , and that south of the tropic 
of Capricorn, the southern temperate zone. The spaces in¬ 
cluded within the polar circles, are called the frigid zones. 
The lines which divide the globe into two equal part^, are 
called the great circles; these are the ecliptic and the equator. 
Those dividing the Earth into smaller parts are called the 
lesser circles; these are the lines dividing the tropics from 
the temperate zones, and the temperate zones from the frigid 
zones, &c. 

842. Horizon.— The horizon is distinguished into the 
sensible and rational. The sensible horizon is that portion 
of the surface of the Earth which bounds our vision, or the 
circle around us, where the sky seems to meet the Earth. 
When the Sun rises, he appears above the sensible horizon, 


What is the ecliptic ? What are the meridian lines 1 On what part of the 
Earth is the torrid zone ? How are the north and south temperate zones 
bounded ? Where are the frigid zones ? Which are the great, and which the 
lesser circles of the Earth ? How is the sensible horizon distinguished from 
the rational ? 




EARTH. 


269 

and when he sets, he sinks below it. The rational horizon 
is an imaginary line passing through the centre of the Earth, 
and dividing it into two equal parts. 

843 Direction of the Ecliptic .—The ecliptic, (758) we 
have already seen, is divided into 360 equal parts, called de¬ 
grees. All circles, however large or small, are divided into 
degrees, minutes, and seconds, in the same manner as the 
ecliptic. 

844. The axis of the ecliptic is an imaginary line passing 
through its centre and perpendicular to its plane. The ex¬ 
tremities of this perpendicular line, are called the poles of the 
ecliptic. 

If the ecliptic, or great plane of the Earth’s orbit, be con¬ 
sidered on the horizon, or parallel with it, and the line of the 
Earth’s axis be inclined to the axis of this plane, or the axis 
of the ecliptic, at an angle of 23£ degrees, it will represent 
the relative positions of the orbit, and the axis of the Earth. 

These positions are, however, merely relative, for if the 
position of the Earth’s axis be represented perpendicular to 
the equator, as A B, fig. 213, then the ecliptic will cross this 
plane obliquely, as in that figure. But when the Earth’s 
orbit is considered as having no inclination, its axis of course 
will have an inclination to the axis of the ecliptic, of 23£ 
degrees. 

As the orbits of all the other planets are inclined to the 
ecliptic, perhaps it is the most natural and convenient method 
to consider this as a horizontal plane, with the equator in¬ 
clined to it, instead of considering the equator on the plane of 
the horizon, as is sometimes done. 

845. Inclination of the Earth’s Axis.— The inclination 
of the Earth’s axis to the axis of its orbit never varies, but 
always makes an angle with it of 23£ degrees, as it moves 
round the Sun. The axis of the Earth is therefore always 
parallel with itself. That is, if a line be drawn through the 
centre of the Earth, in the direction of its axis, and extended 
north and south, beyond the Earth’s diameter, the line so 
produced will always be parallel to the same line, or any 
number of lines, so drawn, when the Earth is in different 
parts of its orbit. 


How are circles divided? What is the axis of the ecliptic? What are the 
poles of the ecliptic ? How many degrees is the axis of the Earth inclined to 
that of the ecliptic ? What is said concerning the relative positions of the 
Earth’s axis and the plane of the ecliptic ? Are the orbits of the other planets 
parallel to the Earth’s orbit, or inclined to it ? What is meant by the Earth’s 
axis Ix.r.. parallel to itself 1 

23 * 



270 


EARTH. 


846. Suppose a rod to be fixed into the flat surface of a 
table, and so inclined as to make an angle with a perpen¬ 
dicular from the table of 23£ degrees. Let this rod repre¬ 
sent the axis of the Earth, and the surface of the table, the 
ecliptic. Now place on the table a lamp, and round the 
lamp hold a wire circle three or four feet in diameter, so that 
it shall be parallel with the plane of the table, and as high 
above it as the flame of the lamp. Having prepared a small 
terrestrial globe, by passing, a wire through it for an axis, 
and letting it project a few inches each way, for the poles, 
take hold of the north pole, and carry it round the circle 
with the poles constantly parallel to the rod rising above 
the table. The rod being inclined 231 degrees from a per¬ 
pendicular, the poles and axis will be inclined in the same 
degree, and thus the axis of the earth will be inclined to that 
of the ecliptic every where in the same degree, and lines 
drawn in the direction of the Earth’s axis will be parallel to 
each other in any part of its orbit. 


Fig. 214. 



This will be understood by fig. 214, where it will be seen, 
that the poles of the Earth, in the several positions of A, B, 
C, and D, being equally inclined, are parallel to each other. 
Supposing the lamp to represent the Sun, and the wire circle 
the Earth’s orbit, the actual position of the Earth, during its 

How does it appear by fig. 214, that the axis of the Earth is parallel to itself, 

in all parts of its orbit? How are the annual and diurnal revolutions of the 
Earth illustrated by fig. 214. 



EARTH. 


271 

annual revolution around the Sun, will be comprehended, and 
if the globe be turned on its axis, while passing round the 
lamp, the diurnal or daily revolution of the Earth will also 
be represented. 


DAY AND NIGHT. 

847. Were the direction of the Earth’s axis perpendicular 
to the plane of its orbit, the days and nights would be of equal 
length all the year, for then just one half of the. Earth, from 
pole to pole, would be enlightened, and at the same time the 
other half would be in darkhess. 

Fig. 215. 





s 


Suppose the line s «?, fig. 215, from the Sun to the Earth 
to be the plane of the Earth’s orbit, and that n s is the axis 
of the Earth perpendicular to it, then it is obvious, that ex¬ 
actly the same points on the Earth would constantly pass 
through the alternate vicissitudes of day and night; for all 
who live on the meridian line between n and s , which line 
crosses the equator at 0 , would see the Sun at the same time, 
and consequently, as the Earth revolves, would pass into the 
dark hemisphere at the same time. Hence in all parts of 
the globe, the days and nights would be of equal length, at 
any given place. 

848. Now it is the inclination of the Earth’s axis, as above 
described, which causes the lengths of the days and nights 
to differ at the same place at different seasons of the year, 
for on reviewing the position of the globe at A, fig 214, it 
will be observed that the line formed by the enlightened and 
dark hemispheres, does not coincide with the line of the axis 
and poles, as in fig. 215, but that the line formed by the 
darkness and the light, extends obliquely across the line of 
the Earth’s axis, so that the north pole is in the light, while 


Explain, by fig. 215, why the days and nights would every where be equal, 
were the axis of the Earth perpendicular to the plane of his orbit? What is 
the cause of the unequal lengths of the days and nights in different parts of the 
world ? 






272 


SEASONS. 


the south is in the dark. In the position A, therefore, an ob¬ 
server at the north pole would see the Sun constantly, while 
another at the south pole would not see it at all. Hence 
those living in the north temperate zone, at the season of the 
year when the Earth is at A, or in the Summer, would have 
long days and short nights, in proportion as they approached 
the polar circle ; while those who live in the south temperate 
zone, at the same time, and when it would be Winter there, 
would have long nights and short days in the same pro¬ 
portion. 

SEASONS OF THE YEAR. 

849. The vicissitudes of the seasons are caused by the an¬ 
nual revolution of the Earth around the Sun , together with the 
inclination of its axis to the plane of its orbit. 

It has already been explained, that the ecliptic is the plane 
of the Earth’s orbit, and is supposed to be placed on a level 
with the Earth’s horizon, and hence, that this plane is con¬ 
sidered the standard, by which the inclination of the lines 
crossing the Earth, and the obliquity of the orbits of the other 
planets, are to be estimated. 

850. The equi- Fi g> 2 i6. 

noctial line, or the 
great circle passing 
round the middle of 
the Earth, is in¬ 
clined to the eclip¬ 
tic, as well as the 
line of the Earth’s 
axis, and hence in ^ 
passing round the 
Sun, the equinoc¬ 
tial line intersects, 
or crosses the eclip¬ 
tic in two places, 
opposite to each 
other. 

Suppose a b , fig. 216, 
and c d , the Earth’s axis. The ecliptic and equator are 
supposed to be seen edgewise, so as to appear like lines in¬ 
stead of circles. Now it will be understood by the figure 
that the inclination of the equator to the ecliptic, (or the Sun’s 



What are the causes which produce the seasons of the year ? In what posi 
tion is the equator, with respect to the ecliptic ? v 










SEASONS. 


273 


apparent annual path through the heavens,) will cause these 
lines, namely, the line of the equator and the line of the eclip> 
tic, to cut, or cross each other, as the Sun makes his appa¬ 
rent annual revolution, and that this intersection will happen 
twice in the year, when the Earth is in the two opposite 
points of her orbit. 

These periods are on the 21st of March, and the 21st of 
September, in each year, and the points at which the Sun is 
seen at these times, are called the equinoctial points. That 
which happens in September is called the autumnal equinox, 
and that which happens in March, the vernal equinox. At 
these seasons, the Sun rises at 6 o’clock and sets at 6 o’clock, 
and the days and nights are equal in length, in every part of 
the globe. 

851. The Solstices.— -The solstices are the points where 
the ecliptic and the equator are at the greatest distance 
from each other. The Earth, in its yearly revolution, pass¬ 
es through each of these points. One is called the Summer , 
and the other the Winter solstice. The Sun is said to enter the 
Summer solstice on the 21st of June; and at this time, in our 
hemisphere, the days are longest, and the nights shortest. 
On the 21st of December, he enters his Winter solstice, 
when the length of the days and nights are reversed from 
what they were in June before, the days being shortest, and 
the nights longest. 

Having learned these explanations, the student will be able 
to understand in what order the seasons succeed each other, 
and the reason why such changes are the effect of the 
Earth’s revolution. 

852. Revolutions of the Earth. —Suppose the Earth, fig. 217, 
to be in her Summer solstice, which takes place on the 21st 
of June. At this period she will be at a, having her north 
pole, », so inclined towards the Sun, that the whole arctic 
circle will be illuminated, and consequently the Sun’s rays 
will extend 23^ degrees, the breadth of the polar circle, be¬ 
yond the north pole. The diurnal revolution, therefore, when 
the Earth is at a, causes no succession of day and night at 
the pole, since the whole frigid zone is within the reach of 
his rays. The people who live within the arctic circle will, 

At what times in the year do the line of the ecliptic and that of the equinox 
intersect each other? What are these points of intersection called ? Which is 
the autumnal, and which the vernal equinox ? At what time does the Sun rise 
and set, when he is in the equinoxes ? What are the solstices ? When the 
Sun enters the Summer solstice, what is said of the length of the days and 
nights ? When does the Sun enter the Winter solstice, and what is the propor¬ 
tion between the length of the days and nights ? At what season of the year is 
the whole arctic circle illuminated ? 



274 


SEASONS. 


consequently, at this time, enjoy perpetual day. During this 
period, just the same proportion of the Earth that is enlight- 

Fig. 217. 



ened in the northern hemisphere, will be in total darkness in 
the opposite region of the southern hemisphere ; so that while 
the people of the north are blessed with perpetual day, those 
of the south are groping in perpetual night. Those who live 
near the arctic circle in the north temperate zone, will during 
the Winter, come, for a few hours, within the regions of night, 
by the Earth’s diurnal revolution; and the greater the dis¬ 
tance from the circle, the longer will be their nights, and the 
shorter their days. Hence, at this season, the days will be 
longer than the nights everywhere between the equator and 
the arctic circle. At the equator, the days and nights will 
be equal, and between the equator and the south polar circle, 
the nights will be longer than the days, in the same propor¬ 
tion as the days are longer than the nights, from the equator 
to the arctic circle. 

Autumnal Equinox .—As the Earth moves round the Sun, 
the line which divides the darkness and the light, gradually 
approaches the poles, till having performed one quarter of her 
yearly journey from the point a, she comes to 6, about the 
21st of September. At this time, the boundary of light and 


At what season is the whole antarctic circle in the dark ? While the people 
near the north pole enjoy perpetual day, what is the situation of those near the 
south pole ? At what season will the days be longer than the nights everywhere 
between the equator and the arctic circle ? At what season will the nights be 
longer than the days in the southern hemisphere ? When will the days and 
nights be equal in all parts of the Earth? 



SEASONS. 


275 

darkness passes through the poles, dividing tne Earth equally 
from east to west; and thus in every part of the world, the 
days and nights are of equal length, the Sun being 12 hours 
alternately above and below the horizon. In this position of 
the Earth, the Sun is said to be in the autumnal equinox. 

In the progress of the Earth from b to s, the light of the 
Sun gradually reaches a little more of the antartic circle. 
The days, therefore, in the northern hemisphere, grow shorter 
at every diurnal revolution, until the 21st of December, when 
the whole arctic circle is involved in total darkness. And 
now, the same places which enjoyed constant day in the 
June before, are involved in perpetual night. At this time, 
the Sun, to those who live in the northern hemisphere, is 
said to be in his Winter solstice; and then the Winter nights 
are just as long as were the Summer days, and the Winter 
days as long as the Summer nights. 

Vernal Equinox .—When the Earth has gone another quar¬ 
ter of her annual journey, and has come to the point of her 
orbit opposite to where she was on the 21st of September, 
which happens on the 21st of March, the line dividing the 
light from the darkness again passes through both poles. 
In this position of the Earth with respect to the Sun, the days 
and nights are again equal all over the world, and the Sun 
is said to be in his vernal equinox. 

From the vernal equinox, as the Earth advances, the 
northern hemisphere enjoys more and more light, while the 
southern falls into the region of darkness, in proportion, so 
that the days north of the equator increase in length, until 
the 21st of June, at which time the Sun is again longest 
above the horizon, and the shortest time below it. 

853. Thus the apparent motion of the Sun from east to 
west, is caused by the real motion of the Earth from west to 
east. If the Earth is in any point of its orbit, the Sun will 
always seem in the opposite point in the heavens. When 
the Earth moves one degree to the west, the Sun seems to 
move the same distance to the east; and when the Earth 
has completed one revolution in its orbit, the Sun appears to 
have completed a revolution through the heavens. Hence 
it follows, that the ecliptic, or the apparent path of the Sun 


At what season of the year is the whole arctic circle involved in darkness ? 
When are the days and nights equal all over the world ? When is the Sun in 
the vernal equinox ? What is the cause of the apparent motion of the Sun from 
east to west ? What is the apparent path of the Sun, but the real path of til* 
Earth? 





SEASONS. 


276 

through the heavens, is the real path of the Earth round the 

354. Sun shines on 180 degrees of the Earth .—It will he 
observed by a careful perusal of the above explanation of the 
seasons, and a close inspection of the figure by which it is 
illustrated, that the Sun constantly shines on a portion of 
the Earth equal to 90 degrees north, and 90 degrees south, 
from his place in the heavens, and consequently, that he 
always enlightens 180 degrees, or one half of the Earth. 
If, therefore, the axis of the Earth were perpendicular to the 
plane of its orbit, the days and nights would every where be 
equal, for as the Earth performs its diurnal revolutions, there 
would be 12 hours day, and 12 hours night. But since the 
inclination of its axis is 231 degrees, the light of the Sun is 
thrown 23£ degrees beyond the north pole ; that is, it enlight¬ 
ens the Earth 23i degrees further in that direction, when the 
north pole is turned towards the Sun, than it would, had the 
Earth’s axis no inclination. Now, as the Sun’s light reaches 
only 90 degrees north or south of his place in the heavens, 
so when the arctic circle is enlightened, the antarctic circle 
must be in the dark; for if the light reaches 23£ degrees 
beyond the north pole, it must fall 23| degrees short of the 
south pole. 

855. As the Earth travels round the Sun, in his yearly 
circuit, this inclination of the poles is alternately towards 
and from him. During our Winter, the north polar region 
is thrown beyond the rays of the Sun, while a corresponding 
portion around the south pole enjoys the Sun’s light. And 
thus, at the poles, there are alternately six months of darkness 
and Winter, and six months of sunshine and Summer. 
While we, in the northern hemisphere, are chilled by the 
cold blasts of Winter, the inhabitants of the southern hemis¬ 
phere are enjoying all the delights of Summer; and while 
we are scorched by the rays of a vertical Sun in June and 
July, our southern neighbors are shivering with the rigors 
of mid-Winter. 

At the equator, no such changes take place. The rays' 
of the Sun, as the Earth passes round him, are vertical twice 
a year at every place between the tropics. Hence, at the 

Had the Earth’s axis no inclination, why would the days and nights always 
be equal? How many degrees does the Sun’s light reach, north and south of 
him, on the Earth ? During our Winter, is the north pole turned to or from the 
Sun ? At the poles, how many days and nights are there in the year ? When 
it is Winter in the northern hemisphere, what is the season in the southern 
hemisphere 7 




SEASONS. 


277 


equator, there are two Summers and no Winter, and as the 
Sun there constantly shines on the same half of the Earth 
in succession, the days and nights are always equal, there 
being 12 hours of light and 12 of darkness. 

856. Velocity of the Earth. —The motion of the Earth 
round the Sun, is at the rate of 68,000 miles in an hour, 
while its motion on its own axis, at the equator, is at the 
rate of about 1,042 miles in the hour. The equator, being 
that part of the Earth most distant from its axis, the motion 
there is more rapid than towards the poles, in proportion to 
its greater distance from the axis of motion. See fig. 14. 
(184.) 

857. The method of ascertaining the velocity of the Earth’s 
motion, both in its orbit and round its axis, is simple, and 
easily understood ; for by knowing the diameter of the Earth’s 
orbit, its circumference is readily found, and as we know 
how long it takes the Earth to perform her yearly circuit, 
we have only to calculate what part of her journey she goes 
through in an hour. By the same principle, the hourly 
rotation of the Earth is as readily ascertained. 

We are insensible to these motions, because not only the 
Earth, but the atmosphere, and all terrestial things, partake 
of the same motion, and there is no change in the relation of 
objects in consequence of it. If we look out at the window 
of a steamboat, when it is in motion, the boat will seem to 
stand still, while the trees and rocks on the shore appear to 
pass rapidly by us. This deception arises from our not 
having any object with which to compare this motion, when 
shut up in the boat; for then every object around us keeps 
the same relative position. And so in respect to the motion 
of the Earth, having nothing with which to compare its 
movement, except the heavenly bodies, when the Earth moves 
in one direction, these objects appear to move in the con¬ 
trary direction. 

CAUSES OF THE HEAT AND COLD OF THE SEA 
SONS. 

858. We have seen that the Earth revolves round the Sun 
in an elliptical orbit, of which the Sun is one of the foci, and 
consequently that the Earth is nearer him, in one part of 
her orbit than in another. From the great difference we 


At what rate does the Earth move around the Sun ? How fast does it move 
around its axis at the equator ? How is the velocity of the Earth ascertained T 
Why are we insensible of the Earth’s motion ? 

24 



278 


SEASONS. 


experience between the heat of Summer and that of Winter, 
we should be led to suppose that the Earth must be much 
nearer the Sun in the hot season than in the cold. But when 
we come to inquire into this subject, and to ascertain the dis¬ 
tance of the Sun at different seasons of the year, we find that 
the great source of heat and light is nearest us during the 
cold of Winter, and at the greatest distance during the heat 
of Summer. 

859. It has been explained, under the article Optics , (702,) 
that the angle of vision depends on the distance at which 
a body of given dimensions is seen. Now, on measuring the 
angular dimension of the Sun, with accurate instruments, 
at different seasons of the year, it has been found that his 
dimensions increase and diminish, and that these variations 
correspond exactly with the supposition that the Earth moves 
in an elliptical orbit. If, for instance, his apparent diameter 
be taken in March, and then again in July, it will be found 
to have diminished, which diminution is only to be accounted 
for, by supposing that he is at a greater distance from the 
observer in July than in March. From July, his angular 
diameter gradually increases, till January, when it again 
diminishes, and continues to diminish, until July. By many 
observations, it is found, that the greatest apparent diameter 
of the Sun, and therefore his least distance from us, is in 
January, and his least diameter, and therefore his greatest 
distance, is in July. The actual difference is about three 
millions of miles, the Sun being that distance further from the 
Earth in July than in January. This, however, is only 
about one-sixtieth of his mean distance from us, and the dif¬ 
ference we should experience in his heat, in consequence of 
this difference of distance, will therefore be very small. 
Perhaps the effect of his proximity to the Earth may dimin¬ 
ish, in some small degree, the severity of Winter. 

860. The heat of Summer, and the cold of Winter, must 
therefore arise from the difference in the meridian altitude of 
the Sun, and in the time of his continuance above the horizon. 
In Summer, the solar rays fall on the Earth, in nearly a per¬ 
pendicular direction, and his powerful heat is then constantly 


At what season of the year is the Sun at the greatest, and at what season the 
least distance, from the Earth ? How is it ascertained that the Earth moves 
in an elliptical orbit, by the appearance of the Sun? When does the Sun 
appear under the greatest apparent diameter, and when under the least? How 
much farther is the Sun from us in July than in January ? What effect does 
this difference produce on the Earth? How is the heat of Summer, and the 
cold of Winter, accounted for ? 



SEASONS. 


279 


accumulated by the long days and short nights of the sea¬ 
son. In Winter, on the contrary, the solar rays fall so ob¬ 
liquely on the Earth, as to produce little warmth, and the 
small effect they do produce during the short days of that 
season, is almost entirely destroyed by the long nights which 
succeed. The difference between the effects of perpend cu- 
lar and oblique rays, seems to depend, in a great measure, on 
the different extent of surface over which they are' spread. 
When the rays of the Sun are made to pass through a con¬ 
vex lens, the heat is increased, because the number of rays 
which naturally covered a large surface, are then made to 
cover a smaller one, so that the power of the glass depends 
on the number of rays thus brought to a focus. If, on the 
contrary, the rays of the Sun are suffered to pass through a 
concave lens, their natural heating power is diminished, 
because they are dispersed, or spread over a wider surface 
than before. 

861. Summer and 
Winter Rays .—Now 
to apply these differ¬ 
ent effects to the 
Summer and Winter 
rays of the Sun, let 
us suppose that the 
rays falling perpen¬ 
dicularly on a given 
extent of surface, im¬ 
part to it a certain 
degree of heat, then 
it is obvious, that if 
the same number of 
rays be spread over 
twice that extent of 
surface, their heat¬ 
ing power would be diminished in proportion, and that only 
half the heat would be imparted. This is the effect produced 
by the Sun’s rays in the Winter. They fall so obliquely on 
the Earth, as to occupy nearly double the space that the 
same number of rays do in the Summer. 

This is illustrated by fig. 218, where the number of rays, 
both in Winter and Summer, are supposed to be the same. 


Fig. 218. 



Why do the perpendicular rays of Summer produce greater effects than the 
oblique rays of Winter? How is this illustrated by the convex and concave 
lenses ? How is the actual difference of the Summer and Winter rays shown ? 




FIGURE OF THE EARTH. 


280 

But, it will be observed, that the Winter rays, owing to their 
oblique direction, are spread over nearly twice as much sur¬ 
face as those of Summer. 

862. It may, however, be remarked, that the hottest sea¬ 
son is not usually at the exact time of the year, when the 
Sun is most vertical, and the days the longest, as is the case 
towards the end of June, but some time afterwards, as in July 
and August. 

To account for this, it must be remembered, that when the 
Sun is nearly vertical, the Earth accumulates more heat by 
day than it gives out at night, and that this accumulation 
continues to increase after the days begin to shorten, and, 
consequently, the greatest elevation of temperature is some 
time after the longest days. For the same reason, the ther¬ 
mometer generally indicates the greatest degree of heat at 
two or three o’clock on each day, and not at twelve o’clock, 
when the Sun’s rays are most powerful. 

FIGURE OF THE EARTH. 

863. Astronomers have proved that all the planets, to¬ 
gether with their satellites, have the shape of the sphere, or 
globe, and hence, by analogy, there was every reason to 
suppose, that the Earth would be found of the same shape; 
and several phenomena tend to prove, beyond all doubt, that 
this is its form. The figure of the Earth is not, however, ex¬ 
actly that of a globe, or ball, because its diameter is about 34 
miles less from pole to pole, than it is at the equator. But 
that its general figure is that of a sphere, or ball, is proved 
by many circumstances. 

864. When one is at sea, or standing on the sea-shore, the 
first part of a ship seen at a distance, is its mast. As the 
vessel advances, the mast rises higher and higher above the 
horizon, and finally the hull, and whole ship, become visible. 
Now, were the Earth’s surface an exact plane, no such ap¬ 
pearance would take place, for we should then see the hull 
long before the mast or rigging, because it is much the largest 
object. 

It will be plain by fig. 219, that were the ship, a, elevated 
so that the hull should be on a horizontal line with the eye, 
the whole ship would be visible, instead of the topmast, there 


Why is not the hottest season of the year at the period when the days are 
longest, and the Sun most vertical ? What is the general figure of the Earth ? 
How much less is the diameter of the Earth at the poles than at the equator ? 
How is the convexity of the Earth proved, by the approach of a ship at sea T 



FIGURE OF THE EARTH. 
Fig. 219. 


281 



being no reason, except the convexity of the earth, why the 
whole ship should not be visible at a, as well as at b. 

We know, for the same reason, that in passing over a hill, 
the tops of the trees are seen, before we can discover the 
ground on which they stand ; and that when a man ap¬ 
proaches from the opposite side of a hill, his head is seen 
before his feet. 

It is a well known fact also, that navigators have set out 
from a particular port, and by sailing continually westward, 
have passed around the Earth, and again reached the port 
from which they sailed. This could never happen, were 
the Earth an extended plain, since then the longer the navi¬ 
gator sailed in one direction, the further he would be from 
home. 

Another proof of the spheroidal form of the Earth, is the 
figure of its shadow on the Moon, during eclipses, which 
shadow is always bounded by a circular line. 

These circumstances prove beyond all doubt, that the form 
of the Earth is globular, but that it is not an exact sphere; 
and that it is depressed or flattened at the poles, is shown by 
the difference in the lengths of pendulums vibrating seconds 
at the poles, and at the equator. 

865. Figure shown by the Pendulum .—Under the article 
pendulum , it was shown that its vibrations depend on the at¬ 
traction of gravitation, and that as the centre of the Earth is 
the centre of this attraction, so the nearer this instrument is 
carried to that point, the stronger will be the attraction, and 
consequently the more frequent its vibrations. 

From a great number of experiments, it has been found 
that a pendulum, which vibrates seconds at the equator, has 
its number of vibrations increased, when it is carried towards 


Explain fig. 219. What other proofs of the globular shape of the Earth are 
mentioned ? How is it proved by the vibrations of the pendulum, that the 
Earth is flattened at the poles? 

24* 




282 


FIGURE OF THE EARTH. 


the poles; and as its number of vibrations depends upon its 
length, a clock which keeps accurate time at the equator, 
must have its pendulum lengthened at the poles. And so, 
on the contrary, a clock going correctly at, or near the poles, 
must have its pendulum shortened, to keep exact time at the 
equator. Hence the force of gravity is greatest at the poles, 
and least at the equator. 

The manner in which 
the figure of the Earth dif¬ 
fers from that of a sphere, 
is represented by fig. 220, 
where n is the north pole, 
and s the south pole, the 
line from one of these points 
to the other, being the axis 
of the Earth, and the line 
crossing this, the equator. 

It will be seen by this fig¬ 
ure, that the surface of the 
Earth at the poles, is nearer 
its centre, than the surface 
at the equator. The actual difference between the polar and 
equatorial diameters is in the proportion of 300 to 301. The 
Earth is therefore called an oblate spheroid , the word oblate 
signifying the reverse of oblong, or shorter in one direction 
than in another. 

866. The compression of the Earth at the poles, and the 
consequent accumulation of matter at the equator, is proba¬ 
bly the effect of its diurnal revolution, while it was in a soft 
or plastic state. If a ball of soft clay, or putty, be made to 
revolve rapidly, by means of a stick passed through its centre, 
as an axis, it will swell out in the middle, or equator, and be 
depressed at the poles, assuming the precise figure of the 
Earth. This figure is the natural and obvious consequence 
of the centrifugal force, which operates to throw the matter 
off, in proportion to its distance from the axis of motion, and 
the rapidity with which the ball is made to revolve. The 
parts about the equator would therefore tend to fly off, and 
leave the other parts, in consequence of the centrifugal force, 
while those about the poles, being near the centre of motion, 



In what proportion is the polar less than the equatorial diameter? What is 
the Earth called in reference to this figure ? How is it supposed that it came 
to have this form ? How is the form of the Earth illustrated by experiment ? 
Explain the reason why a plastic ball will swell at the equator, when made to 
evolve! 






TIME. 


283 

would receive a much smaller impulse. Consequently, the 
ball would swell, or bulge out at the equator, which would 
produce a corresponding depression at the poles. 

867. The weight of a body at the poles is found to be 
greater than at the equator, not only because the poles are 
nearer the centre of the Earth than the equator, but because 
the centrifugal force there tends to lessen its gravity. The 
wheels of machines, which revolve with the greatest rapidity, 
are made in the strongest manner, otherwise, they will fly in 
pieces, the centrifugal force not only overcoming the gravity, 
but the cohesion of their parts. 

868. It has been found, by calculation, that if the Earth 
turned over once in 84 minutes and 43 seconds, the centrifu¬ 
gal force at the equator would be equal to the power of 
gravity there, and that bodies would entirely lose their 
weight. If the Earth revolved more rapidly than this, all 
the buildings, rocks, mountains, and men, at the equator, 
would not only lose their weight, but would fly away, and 
leave the Earth. 

SOLAR AND SIDERIAL TIME. 

869. The stars appear to go round the Earth in 23 hours, 
56 minutes, and 4 seconds, while the Sun appears to perform 
the same revolution in 24 hours, so that the stars gain 3 
minutes and 56 seconds upon the Sun every day. In a year, 
this amounts to a day, or to the time taken by the Earth to 
perform one diurnal revolution. It therefore happens, that 
when time is measured by the stars, there are 366 days in 
the year, or 366 diurnal revolutions of the Earth; while, if 
measured by the Sun from one meridian to another, there 
are only 365 whole days in the year. The former are called 
the siderial , and the latter solar days. 

To account for this difference, we must remember that the 
Earth, while she performs her daily revolutions, is constantly 
advancing in her orbit, and that, therefore, at 12 o’clock 
to-day she is not precisely at the same place in respect to the 
Sun, that she was at 12 o’clock yesterday, or will be to-mor¬ 
row. But the fixed stars are at such an amazing distance 


What two causes render the weights of bodies less at the equator than at the 
poles ,? What would be the consequence on the weights of bodies at the equa¬ 
tor, did the Earth turn over once in 84 minutes and 43 seconds ? The stars 
appear to move round the Earth in less time than the Sun ; what does the dif¬ 
ference amount to in a year ? What is the year measured by a star called ? 
What is that measured by the Sun called ? How is the difference in time be 
tween the solar and siderial year accounted for ? 



284 


TIME. 


from us, that the Earth’s orbit, in respect to them, is but a 
point; and, therefore, as the Earth’s diurnal motion is per¬ 
fectly uniform, she revolves from any given star to the same 
star again in exactly the same period of absolute time. The 
orbit of the Earth, were it a solid mass, instead of an ima¬ 
ginary circle, would have no appreciable length or breadth, 
when seen from a fixed star, and therefore, whether the Earth 
performed her diurnal revolutions at a particular station, or 
while passing round in her orbit, would make no appreciable 
difference with respect to the star. Hence the same star, at 
every complete daily revolution of the Earth, appears pre¬ 
cisely in the same direction at all seasons of the year. The 
Moon, for instance, would appear at exactly the same point, 
to a person who walks round a circle of a hundred yards, in 
diameter, and for the same reason a star appears in the same 
direction from all parts of the Earth’s orbit, though 190 
millions of miles in diameter. 

870. If the Earth had only a diurnal motion, her revolu¬ 
tion, in respect to the Sun, would coincide exactly with the 
same revolution in respect to the stars; but while she is 
making one revolution on her axis towards the east, she 
advances in the same direction about one degree in her orbit, 
so that to bring the same meridian towards the Sun, she must 
make a little more than one entire revolution. 

To make this plain, suppose the Sun, s, fig. 221, to be ex¬ 
actly on a meridian line marked <?, on the Earth i, on a 
given day. On the next day, the Earth, instead of being at 
-A, as on the day before, advances in its orbit to B f and in 


Fig 221. 



The Earth’s orbit is but a point, in reference to a star j how is this illustrated ? 





TIME. 


285 


the mean time having completed her revolution, in respect to 
a star, the same meridian line is not brought under the Sun, 
as on the day before, but falls short of it, as at e, so that the 
Earth has to perform more than a revolution, by the distance 
from e to o, in order to bring the same meridian again under 
the Sun. So on the next day, when the Earth is at C, she 
must again complete more than two revolutions, since leav¬ 
ing A, by the space from e to a, before it will again be noon 
at e. 

871. Thus, it is obvious, that the Earth must complete one 
revolution, and a portion of a second revolution, equal to the 
space she has advanced in her orbit, in order to bring the 
same meridian back again to the Sun. This small portion 
of a second revolution amounts daily to the 365th part of her 
circumference, and therefore, at the end of the year, to one 
entire rotation, and hence in 365 days, the Earth actually 
turns on her axis 366 times. Thus, as one complete rotation 
forms a siderial day, there must, in the year, be one siderial, 
more than there are solar days, one rotation of the Earth, 
with respect to the Sun, being lost, by the Earth's yearly 
revolution. The same loss of a day happens to a traveller, 
who, in passing round the Earth towards the west, reckons 
his time by the rising and setting of the Sun. If he passes 
round towards the east, he will gain a day for the same 
reason. 

EQUATION OF TIME. 

872. As the motion of the Earth about its axis is perfectly 
uniform, the siderial days, as we have already seen, are ex¬ 
actly of the same length, in all parts of the year. But as 
the orbit of the Earth, or the apparent path of the Sun, is in¬ 
clined to the Earth’s axis, and as the Earth moves with dif¬ 
ferent velocities in different parts of its orbit, the solar, or 
natural days, are sometimes greater and sometimes less than 
24 hours, as shown by an accurate clock. The consequence 
is, that a true sun-dial, or noon mark, and a true time piece, 
agree with each other only a few times in a year. The dif¬ 
ference between the sun-dial and clock, thus shown, is call¬ 
ed the equation of time. 

The difference between the Sun and a well regulated 

Had the Earth only a diurnal revolution, would the siderial and solar time 
agree ? Show by fig. 221, how siderial differs from solar time ? Why does not 
the Earth turn the same meridian to the Sun at the same time every day ? 
How many times does the Earth turn on her axis in a year ? Why does she 
turn more times than there are days in the year? Why are the solar day* 
sometimes greater, and sometimes less, than 24 hours ? 



TIME. 


2*6 

clock, thus arises from two causes, the inclination of the 
Earth’s axis to the ecliptic, and the elliptical fprm of the 
Earth’s orbit. 

873. That the Earth moves in an ellipse, and that its mo¬ 
tion is more rapid sometimes than at others, as well as that 
the Earth’s axis is inclined to the ecliptic, have already been 
explained and illustrated. It remains, therefore, to show how 
these two combined causes, the elliptical form of the orbit, 
and the inclination of the axis, produce the disagreement be¬ 
tween the Sun and clock. In this explanation, we must 
consider the Sun as moving around the ecliptic, while the 
Earth revolves on her axis. 

874. Mean Time.—Equal , or mean time, is that which is 
reckoned by a clock, supposed to indicate exactly 24 hours, 
from 12 Vclock on one day, to 12 o’clock on the next day 
Apparent ame, is that which is measured by the apparent 
motion of the Sun in the heavens, as indicated by a meridian 
line, or sun-dial. 

875. Were the Earth’s orbit a perfect circle, fig. 222, and 
her axis perpendicular to the plane of this orbit, the days 
would be of a uniform length, and there would be no differ¬ 
ence between the clock and the Sun; both would indicate 
12 o’clock at the same time, on every day in the year. But 
on account of the inclination of the Earth’s axis to the eclip¬ 
tic, unequal portions of the Sun’s apparent path through the 
heavens will pass any meridian in equal times. This may 
be readily explained to the pupil, by means of an artificial 
globe; but perhaps it will be understood by the following 
diagram. 

Let A N B S, fig. 222, be the concave of the heavens, in 
the centre of which is the Earth. I <et the line A B, be the 
equator, extending through the Earth and the heavens, and 
let A, a, b , C, c, and d, be the ecliptic, or the apparent path 
of the Sun through the heavens. Also, let A, 1,2, 3, 4, 5, 
be equal distances on the equator, and A, a, 6, C, c, and d , 
equal portions of the ecliptic, corresponding with A 1, 2, 3, 
4, and 5. Now we will suppose, that there are two sunsj 
namely, a false, and a real one ; that the false one passes 
through the celestial equator, which is only an extension of 
the Earth’s equator to the heavens; while the real Sun has 

What is the difference between the time of a sun-dial and a clock called 7 
What are the causes of the difference between the Sun and clock 7 In ex 
plaining equation of time, what motion is considered as belonging to the Sun, 
and what motion to the Earth 7 What is equal, or mean time 7 What is anna 
rent time 7 



TIME. 


287 


an apparent revo- Fig. 222. 

lution through the 
ecliptic; and that 
they both start 
from the point A, 
at the same in¬ 
stant. The false 
Sun is supposed to 
pass through the 
celestial equator 
in the same time 
that the real one 
passes through 
the ecliptic, but 
not through the 
same meridians at 
the same time, so 
that the false Sun 
arrives at the 
points 1, 2, 3, 4, and 5, at the time when the real Sun ar¬ 
rives at the points a, b , C, and c. When the two suns 
were at A, the starting point, they were both on the same 
meridian, but when the fictitious Sun comes to 1, and the real 
Sun to c, they are not in the same meridian, but the real Sun 
is westward of the fictitious one, the real Sun being at a 
while the false Sun is on the meridian 1, consequently, as 
the Earth turns on its axis from west to east, any particular 
place will come under the Sun’s real meridian, sooner than 
under the fictitious Sun’s meridian; that is, it will be 12 
o’clock by the true Sun, before it is 12 o’clock by the false 
Sun, or by a true clock; but were the true Sun in place of 
the false one, the Sun and clock would agree. While th* 
true Sun is passing through that quarter of its orbit, from 
to C, and the fictitious Sun from 1 to 3, it will always be 
noon by the true Sun before it is noon by the false Sun, and 
during this period, the Sun will be faster than the clock. 

When the true Sun arrives at C, and the false one at 3, 
they are both on the same meridian, and the Sun and clock 
agree. But while the real Sun is passing from C to B, and 



In fig. 222, which is the celestial equator, and which the ecliptic ? Througl 
which of these circles does the false, and through which does the true Sui 
pass? When the real Sun arrives to a, and the false one to 1, are they both 
on, the same meridian ? Which is then most westward ? When the two suns 
are at ], and o, why will any meridian come first under the real Sun? Were 
th* *vue Sun in place of the false one, why would the Sun and clock agree ? 














288 


TIME. 


the false one from 3 to B, any meridian comes later under the 
true Sun than it does under the false, and then it is noon b} r 
the Sun after it is noon by the clock, and the Sun is then said 
to be slower than the clock. At B, both suns are again on 
the same meridian, and then again the Sun and clock agree. 

We have thus followed the real Sun through one half of 
his true apparent place in the heavens, and the false one 
through half the celestial equator, and have seen that the 
two suns, since leaving the point A, have been only twice on 
the same meridian at the same time. It has been supposed that 
the two suns passed through equal arcs, in equal times, the 
real Sun through the ecliptic, and the false one through the 
equator. The place of the false Sun may be considered as 
representing the place where the real Sun would be, in case 
the Earth’s axis had no inclination, and consequently it 
agrees with the clock every 24 hours. But the true Sun, as 
he passes round in the ecliptic, comes to the same meridian, 
sometimes sooner, and sometimes later, and in passing around 
the other half of the ecliptic, or in the other half year, the 
same variations succeed each other. 

Fast and Slow o'clock .—The two Suns are supposed to 
depart from the point A, on the 20th of March, at whicl^. 
time the Sun and clock coincide. From this time, the Sun 
is faster than the clock, until the two Suns come together at 
the point C, which is on the 21st of June, when the Sun and 
clock again agree. From this period the Sun is slower than 
the clock, until the 23d of September, and faster again until 
the 21st of December, at which time they agree as before. 

We have thus seen how the inclination of the Earth’s axis, 
and the consequent obliquity of the equator to the ecliptic, 
causes the Sun and clock to disagree, and on what days they 
would coincide, provided no other cause interfered with their 
agreement. But although the inclination of the Earth’s 
axis would bring the Sun and clock together on the above- 
mentioned days, yet this agreement is counteracted by an¬ 
other cause, which is the elliptical form of the Earth’s orbit, 
and though the Sun and clock do agree four times in the 
year, it is not on any of the days above-mentioned. 

It has been shown by fig. 212, that the Earth moves more 
rapidly in one part of its orbit than in another. When it is 

While the suns are passing from A to C, and from 1 to 3, will the Sun be 
faster or slower than the clock ? When the two suns are at C, and 3, why will 
the Sun and clock agree ? While the real Sun is passing from B to C, which 
is fastest, the clock or Sun ? What does the place of the false Sun represent, 
in fig. 222T K 



TIME. 


289 


nearest the Sun, which is in the Winter, its velocity is greater 
than when it is most remote from him, as in Summer. Were 
the Earth’s orbit a perfect circle, the Sun and clock would 
coincide on the days above specified, because then the only 
disagreement would arise from the inclination of the Earth’s 
axis. But since the Earth’s distance from the Sun is con¬ 
stantly changing, her rate of velocity also changes, and she 
passes through unequal portions of her orbit in equal times. 
Hence, on some days, she passes through a greater portion 
of it than on others, and thus this becomes another cause of 
the inequality of the Sun’s apparent motion. 

The elliptical form of the Earth’s orbit would prevent the 
coincidence of the Sun and clock at all times, except when 
the Earth is at the greatest distance from the Sun, which 
happens on the 1st of July, and when she is at the least dis¬ 
tance from him, which happens on the 1st of January. As 
the Earth moves faster in the Winter than in the Summer, 
from this cause, the Sun would be faster than the clock from 
the 1st of July to the 1st of January, and then slower than 
the clock from the 1st of January to the 1st of July. 

876. When the Sun and Clock agree .—We have now 
explained, separately, the two causes which prevent the coin¬ 
cidence of the Sun and clock. By the first cause which is 
the inclination of the Earth’s axis, they would agree four 
times in the year, and by the second cause, the irregularity 
of the Earth’s motion, they would coincide only twice in the 
year. 

Now, these two causes counteract the effects of each 
other, so that the Sun and clock do not coincide on any of 
the days, when either cause, taken singly, would make an 
agreement between them. The Sun and clock, therefore, 
are together, only when the two causes balance each other ; 
that is, when one cause so counteracts the other, as to make 
a mutual agreement between them. This effect is produced 
four times in the year; namely, on the 15th of April, 15th 
of June, 31st of August, and 24th of December. On these 


The inclination of the Earth’s axis would make the Sun and clock agree in 
March, <md the other months above named: why then do they not actually 
agree at those times ? Were the Earth’s orbit a perfect circle, on what days 
would the Sun and clock agree ? How does the form of the Earth’s orbit 
interfere with the agreement of the Sun and clock on those days ? At what 
times would the form of the Earth’s orbit bring the Sun and clock to agree 7 
The inclination of the Earth’s axis would make the Sun and clock agree four 
times in the year, and the form of the Earth’s orbit would make them agree 
twice in the year; now show the reason why they do not agree from these 
causes, on the above-mentioned days, and why they do agree on other days 7 

25 



MOON 


290 

days. the Sun, and a clock keeping exact time, coincide, and 
on no others. The greatest difference between the Sun and 
clock, or between the apparent and mean time, is 16£ min¬ 
utes, which takes place about the 1 st of November. 

THE MOON. 

877. While the Earth revolves round the Sun, the Moon 
revolves round the Earth, completing her revolution once in 
27 days, 7 hours, and 43 minutes, and at the distance of 
240,000 miles from the Earth. The periods of the Moon’s 
change, that is, from new Moon to new Moon again, is 29 
days, 12 hours, and 44 minutes. 

878. The time of the Moon’s revolution round the Earth 
is called her periodical month; and the time from change to 
change is called her synodical month. If the Earth had no 
annual motion, these two periods would be equal, but because 
the Earth goes forward in her orbit, while the Moon goes 
round the Earth, the Moon must go as much farther, from 
change to change, to make these periods equal, as the Earth 
goes forward during that time, which is more than the 
twelfth part of her orbit, there being more than twelve lunar 
periods in the year. 

879. Illustration by the Hands of a Watch .—These two 
revolutions may be familiarly illustrated by the motions of 
the hour and minute hands of a watch. Let us suppose 
the 12 hours marked on the dial plate of a watch to repre¬ 
sent the 12 signs of the zodiac through which the Sun seems 
to pass in his yearly revolution, while the hour hand of the 
watch represents the Sun, and the minute hand the Moon. 
Then, as the hour hand goes around the dial plate once in 
12 hours, so the Sun apparently goes around the zodiac once 
in twelve months; and as the minute hand makes 12 revo¬ 
lutions to one of the hour hand, so the Moon makes 12 revo¬ 
lutions to one of the Sun. But the Moon, or minute hand, 
must go more than once round, from any point on the circle, 
where it last came in conjunction with the Sun, or hour hand, 
to overtake it again, since the hour hand will have moved 
forward of the place where it was last overtaken, and conse¬ 
quently the next conjunction must be forward of the place 


On what days do the Sun and clock agree ? What is the period of the Moon’s 
revolution round the Earth ? What is the period from new Moon to new 
Moon again? What are these two periods called? Why are not the periodi¬ 
cal and synodical months equal ? How are these two revolutions of the Moor 
illustrated by the two hands of a Watch ? 



MOON. 


291 


where the last happened. During an hour, the hour hand 
describes the twelfth part of the circle, but the minute hand 
has not only to go round the whole circle in an hour, but also 
such a portion of it,'as the hour hand has moved forward 
since they last met. Thus, at 12 o’clock, the hands are in 
conjunction ; the next conjunction is 5 minutes 27 seconds 
past I o’clock; the next, 10 min. 54 sec. past II o’clock ; 
the third, 16 min. 21 sec. past III; the 4th, 21 min. 49 sec. 
past IV ; the 5th, 27 min. 10 sec. past V ; the 6th, 32 min. 
43 sec. past VI; the 7th, 38 min. 10 sec. past VII; the 8th, 
43 min. 38 sec. past VIII; t'he 9th, 49 min. 5 sec. past IX; 
the 10th, 54 min. 32 sec. past X; and the next conjunction 
is at XII. 

Now although the Moon passes around the Earth in 27 
days 7 hours and 43 minutes, yet her change does not take 
place at the end of this period, because her changes are not 
occasioned by her revolutions alone, but by her coming peri¬ 
odically into the same position in respect to the Sun. At her 
change, she is in conjunction with the Sun, when she is not 
seen at all, and at this time astronomers call it new Moon , though 
generally, we say it is new Moon two days afterwards, 
when a small part of her face is to be seen. The reason 
why there is not a new Moon at the end of 27 days, will be 
obvious, from the motions of the hands of a watch ; for we 
see that more than a revolution of the minute hand is required 
to bring it again in the same position with the hour hand, by 
about the twelfth part of the circle. 

The same principle is true in respect to the Moon ; for as 
the Earth advances in its orbit, it takes the Moon 2 days 5 
hours and 1 minute longer to come again in conjunction 
with the Sun, than it does to make her monthly revolution 
round the Earth ; and this 2 days 5 hours and 1 minute 
being added to 27 days 7 hours and 43 minutes, the time of 
the periodical revolution makes 29 days 12.hours and 44 
minutes, the period of her synodical revolution. 

880. We only see one side of the Moon. —The Moon al¬ 
ways presents the same side, or face, towards the Earth, and 
hence it is evident that she turns on her axis but once, while 
she is performing one revolution round the Earth, so that the 


Mention the time of several conjunctions between the two hands of a watch. 
Why do not the Moon’s changes take place at the periods of her revolution 
around the Earth ? How much longer does it take the Moon to come again m 
conjunction with the Sun, than it does to perform her periodical revolution ? 
How is it proved that the Moon makes but one revolution on her axis, as she 
passes around the Earth ? 




MOON. 


00*2 

inhabitants of the Moon have but one day, and one night in 
the course of a lunar month. 

One half of the Moon is never in the dark, because when 
this half is not enlightened by the Sun, a strong light is re¬ 
flected to her from the Earth, during the Sun’s absence. 
The other half of the Moon enjoys alternately two weeks of 
the Sun’s light, and two weeks of total darkness. 

The Moon is a globe, like our Earth, and like the Earth, 
shines only by the light reflected from the Sun ; therefore, 
while that half of her which is turned towards the Sun is en¬ 
lightened, the other half is in darkness. Did the Moon shine 
by her own light, she would be constantly visible to us, for 
then, being an orb, and every part illuminated, we should 
see her constantly full and round, as we do the Sun. 

881. Phases of the Moon .—One of the most interesting 
circumstances to us, respecting the Moon, is, the constant 
changes which she undergoes, in her passage around the 
Earth. When she first appears, a day or two after her 
change, we can see only a small portion of her enlightened 
side, which is in the form of a crescent; and at this time she 
is commonly called new Moon. From this period she goes 
on increasing, or showing more and more of her face every 
evening, until at last she becomes round, and her face is fully 
illuminated. She then begins again to decrease, by appa¬ 
rently losing a small section of her face, and the next eve¬ 
ning another small section from the same part, and so on, 
decreasing a little every day, until she entirely disappears ; 
and having been absent a day or two, re-appears in the form 
of a crescent, or new Moon, as before. 

882. When the Moon disappears, she is said to be in con¬ 
junction, that is, she is in the same direction from us with 
the Sun. When she is full, she is said to be in opposition , 
that is, she is in that part of the heavens opposite to the Sun, 
as seen by us. 

883. The different appearances of the Moon from new to 
full, and from full to change , are owing to her presenting dif¬ 
ferent portions of her enlightened surface towards us at differ¬ 
ent times. These appearances are called phases of the Moon, 
and are easily accounted for, and understood, by the follow¬ 
ing figure. 


One half of the Moon is never in the dark; explain why this is so. How 
long is the day and night at the other half? How is it shown that the Moon 
shines only by reflected light? When is the Moon said to be in conjunction 
with the Sun, and when in opposition to the Sun t What are the phases of 
the Moon ? 



MOON. 293 

Fig. 223. 



Let S, fig. 223, be the Sun, E the Earth, and A, B, C, D, 
F , the Moon in different parts of her orbit. Now when the 
Moon changes, or is in conjunction with the Sun, as at A, 
her dark side is turned towards the Earth, and she is invisible, 
as represented at a. The Sun always shines on one half of 
the Moon, in every direction, as represented at A and F, on 
the inner circle ; but we at the Earth can see only such por¬ 
tions of the enlightened half as are turned towards us. After 
her change, when she has moved from A to Z?, a small part 
of her illuminated side comes in sight, and she appears horned, 
as at b , and is then called the new Moon. When she ar¬ 
rives at C, several days afterwards, one half of her disc is 
visible, and she appears as at c, her appearance being the 
same in both circles. At this point she is said to be in her 
first quarter , because she has passed through a quarter of 
her orbit, and is 90 degrees from the place of her conjunction 
with the Sun. At D, she shows us still more of her enlight¬ 
ened side, and is then said to appear gibbous as at d. When 
she comes to F, her whole enlightened side is turned towards 
the Earth, and she appears in all the splendor of a full Moon. 
During the other half of her revolution, she daily shows less 
and less of her illuminated side, until she again becomes in¬ 
visible by her conjunction with the Sun. Thus in passing 
from her conjunction a, to her full, e, the Moon appears every 
day to increase, while in going from her full to her con¬ 
junction again, she appears to us constantly to decrease, but 
as seen from the Sun, she appears always full. 


Describe fig. 223, and show how the Moon passes from change to full, and 
from full to change. What is said concerning the phases of the Earth, as seen 
from the Moon ? 

25* 





MOOIV 


294 

884. How the Earth appears at the Moon .—The Earth, 
seen by the inhabitants of the Moon, exhibits the same 
phases that the Moon does to us, but in a contrary order. 
When the Moon is in her conjunction, and consequently 
invisible to us, the Earth appears full to the people of the 
Moon, and when the Moon is full to us, the Earth is dark to 
them. 

The Earth appears thirteen times larger to the lunarians 
than the Moon does to us. As the Moon always keeps the 
same side towards the Earth, and turns on her axis only as 
she moves round the Earth, we never see her opposite side. 
Consequently the lunarians who live on the opposite side to 
us never see the Earth at all. To those who live on the 
middle of the side next to us, our Earth is always visible, 
and directly over head, turning on its axis nearly thirty times 
as rapidly as the Moon, for she turns only once in about 
thirty days. A lunar astronomer, who should happen to 
live directly opposite to that side of the Moon which is next 
to us, would have to travel a quarter of the circumference of 
the Moon, or about 1,500 miles, to see our Earth above the 
horizon, and if he had the curiosity to see such a glorious 
orb, in its full splendor over his head, he must travel 3,000 
miles. But if his curiosity equalled that of the terrestrials, 
he would be amply compensated by beholding so glorious a 
nocturnal luminary, a Moon thirteen times as large as ours. 

885. That the Earth shines upon the Moon, as the Moon 
does upon us, is proved by the fact that the outline of her 
whole disc may be seen, when only a part of it is enlightened 
by the Sun. Thus when the sky is clear, and the Moon 
only two or three days old, it is not uncommon to see the 
brilliant new Moon, with her horns enlightened by the Sun, 
and at the same time the old Moon faintly illuminated by 
reflection from the Earth. This phenomenon is sometimes 
called u the old Moon in the new Moon’s arms.” 

It was a disputed point among former astronomers, whether 
the Moon has an atmosphere; but the more recent discov¬ 
eries have decided that she has an atmosphere, though there 
is reason to believe that it is much less dense than ours. 

886. Surface of the Moon .—When the Moon’s surface is 
examined through a telescope, it is found to be wonderfully 


When does the Earth appear full at the Moon ? When is the Earth in her 
change, to the people of the Moon? Why do those who live on one side of the 
Moon neve, see the Earth ? How is it known that the Earth shines upon the 
Moon, as the Moon does upon us ? What is said concerning the Moon’s at¬ 
mosphere? 



ECLIPSES. 


295 

diversified, for besides the dark spots perceptible to the naked 
eye, there are seen extensive valleys, and long ridges of 
highly elevated mountains. 

Some of these mountains, according to Dr. Herschel, are 
4 miles high, while hollows more than 3 miles deep, and al¬ 
most exactly circular, appear excavated on the plains. As- 
stronomers have been at vast labor to enumerate, figure, and 
describe the mountains and spots on the surface of the Moon, 
so that the latitude and longitude of about 100 spots have 
been ascertained, and their names, shapes, and relative posi¬ 
tions given. A still greater number of mountains have been 
named, and their heights and the length of their bases de¬ 
tailed. 

887. The deep caverns, and broken appearance of the 
Moon’s surface, long since induced astronomers to believe 
that such effects were produced by volcanoes, and more re¬ 
cent discoveries have seemed to prove that this suggestion 
was not without foundation. Dr. Herschel saw with his 
telescope, what appeared to him three volcanoes in the Moon, 
two of which were nearly extinct, but the third was in the 
actual state of eruption, throwing out fire, or other luminous 
matter, in vast quantities. 

888. It was formerly believed that several large spots, 
which appeared to have plane surfaces, were seas, or lakes, 
and that a part of the Moon’s surface was covered with 
water, like that of our Earth. But it has been found, on 
closely observing these spots, when they were in such a 
position as to reflect the Sun’s light to the Earth, had they 
been water, that no such reflection took place. It has also 
been found, that when these spots were turned in a certain 
position, their surfaces appeared rough, and uneven ; a cer¬ 
tain indication that they are not water. These circum¬ 
stances, together with the fact, that the Moon’s surface is 
never obscured by mist or vapor, arising from the evaporation 
of water from her surface, have induced astronomers to be¬ 
lieve, that the Moon has neither seas, lakes, or rivers, and in¬ 
deed that no water exists there. 

ECLIPSES. 

889. Every planet and satellite in the solar system is illu¬ 
minated by the Sun , and hence they cast shadows in the direction 

How high are some of the mountains, and how deep the caverns of the Moon ? 
What is said concerning the volcanoes of the Moon ? What is supposed con¬ 
cerning the lakes and seas of the Moon? On what grounds is it supposed that 
there is no water at the Moon ? 



296 


ECLIPSES. 


opposite to him , just as the shadow of a man reaches from the 
Sun. A shadow is nothing more than the interception of the 
rays of light by an opaque body. The Earth always mattes 
a shadow, which reaches to an immense distance into open 
space, in the direction opposite to the Sun. When the Earth, 
turning on its axis, carries us out of the sphere of the Sun’s 
light, we say it is sunset , and then we pass into the Earth’s 
shadow, and night comes on. When the Earth turns half 
round from this point, and we again emerge out of the 
Earth’s shadow, we say, the sun rises , and then day begins. 

890. Now an eclipse of the Moon is nothing more than 
her falling into the shadow of the Earth. The Moon, having 
no light of her own, is thus darkened, and we say she is 
eclipsed. The shadow of the Moon also reaches to a great 
distance from her. We know that it reaches at least 240,000 
miles, because it sometimes reaches the Earth. An eclipse 
of the Sun is occasioned whenever the Earth falls into the 
shadow of the Moon. Hence, in eclipses, whether of the 
Sun or Moon, the two planets and the Sun must be nearly in 
a straight line with respect to each other. In eclipses of the 
Moon, the Earth is between the Sun and Moon, and in 
eclipses of the Sun, the Moon is between the Earth and Sun. 

891. If the Moon went around the Sun in the same plane 
with the Earth, that is, were the Moon’s orbit on the plane 
of the ecliptic, there would happen an Eclipse of the Sun at 
every conjunction of the Sun and Moon, or at the time of 
every new Moon. But at these conjunctions the Moon does 
not come exactly between the Earth and Sun, because the 
orbit of the Moon is inclined to the ecliptic at an angle of 5^ 
degrees. Did the planes of the orbits of the Earth and 
Moon coincide, there would be an eclipse of the Moon at 
every full, for then the Moon would pass exactly through the 
Earth’s shadow. 

892. Moon's Nodes .—One half of the Moon’s orbit being 
elevated, 5\ degrees above the ecliptic, the other half is 
d( pressed as much below it, and thus the Moon’s orbit crosses 
that of the Earth in two opposite points, called the Moon’s 
nodes. 


What is a shadow ? When do we say it is sunset, and when do we say it 
is sunrise ? What occasions an eclipse of the Moon ? What causes eclipses 
of the Sun? In eclipses of the Moon, what planet is between the Sun and 
Moon ? In eclipses of the Sun, what planet is between the Sun and Eaith i 
Why is there not an eclipse of the Sun at every conjunction of the Sun and 
Moon ? How many degrees is the Moon’s orbit inclined to that of the Earth 1 
What are the nodes of the Moon ? 



ECLIPSES. 


297 


As the nodes of the Moon are the points where she crosses 
the ecliptic, she must be half the time above, and the other 
half below these points. The node in which she crosses 
the plane of the ecliptic upward, or towards the north, is 
called her ascending node. That in which she crosses the 
same plane downward, or towards the south, is called her 
descending node. 

The Moon’s orbit, like those of the other planets, is ellip¬ 
tical, so that she is sometimes nearer the Earth than at others. 
When she is in that part of her orbit, at the greatest dis¬ 
tance from the Earth, she ip said to be in her apogee , and 
when at her least distance from the Earth, she is in her 
perigee. 

893. Eclipses can only happen at the time when the Moon 
is at, or near, one of her nodes, for at no other time is she 
near the plane of the Earth’s orbit; and since the Earth is 
always in this plane, the Moon must be at, or near it, also, 
in order to bring the two planets and the Sun in the same 
right line, without which no eclipse can happen. 

894. The reason why eclipses do not happen oftener, and 
at regular periods, is because a node of the Moon is usually 
only twice, and never more than three times in the year, 
presented towards the Sun. The average number of total 
eclipses of both luminaries, in a century, is about thirty, and 
the average number of total and partial, in a year, about 
four. There may be seven eclipses in a year, including 
those of both luminaries, and there may be only two. When 
there are only two, they are both of the Sun. 

When the Moon is within 16£ degrees of her node, at the 
time of her change, she is so near the ecliptic, that the Sun 
may be more or less eclipsed, and when she is within 12 de¬ 
grees of her node, at the time of her full, the Moon will be 
more or less eclipsed. 

895. But the Moon is more frequently within 16? degrees 
of her node at the time of her change, than she is within 12 
degrees at the time of her full, and consequently there will 
be a greater number of solar, than of lunar eclipses, in a 
course of years. Yet more lunar eclipses will be visible, at 
any one place on the Earth, than solar, because the Sun, 


What is meant by the ascending and descending nodes of the Moon ? What 
is the Moon’s apogee, and what her perigee ? Why must the Moon be at, or 
near, one of her nodes, to occasion an eclipse? Why do not eclipses happen 
often, and at regular periods ? What is the greatest, and what the least num¬ 
ber of eclipses that can happen in a year? Why will there be more solar than 
lunar eclipses in the course of years ? 



298 


ECLIPSES. 


being so much larger than the Earth, or Moon, the shadow 
of these bodies must terminate in a point, and this point of 
the Moon’s shadow never covers but a small portion of the 
Earth’s surface, while lunar eclipses are visible over a whole 
hemisphere, and as the Earth turns on its axis, are therefore 
visible to more than half the Earth. This will be obvious 
by figs. 224 and 225, where it will be observed that an 
eclipse of the Moon may be seen wherever the Moon is visi¬ 
ble, while an eclipse of the Sun will be total only to those 
who live within the space covered by the Moon’s dark shadow. 

896. Lunar Eclipses. — When the Moon falls into the 
shadow of the Earth, the rays of the Sun are intercepted, or 
hid from her, and she then becomes eclipsed. 

When the Earth’s shadow covers only a part of her face, 
as seen by us, she suffers only a partial eclipse, one part of 
her disc being obscured, while the other part reflects the Sun’s 
light. But when her whole surface is obscured by the 
Earth’s shadow, she then suffers a total eclipse, and of a 
duration proportionate to the distance she passes through the 
Earth’s shadow. 

897. Fig. 224 represents a total lunar eclipse; the Moon be¬ 



ing in the midst of the Earth’s shadow. Now it will be appa¬ 
rent that in the situation of the Sun, Earth, and Moon, as 
represented in the figure, this eclipse will be visible from all 
parts of that hemisphere of the Earth which is next the 
Moon, and that the Moon’s disc will be equally obscured, 
from whatever point it is seen. When the moon passes 
through only a part of the Earth’s shadow, then she suffers 
only a partial eclipse, but this is also visible from the whole 
hemisphere next the Moon. It will be remembered that lu¬ 
nar eclipses happen only at full Moon, the Sun and Moon 
being in opposition, and the Earth between them. 


Why will more lunar, than solar eclipses be visible at any one place 7 Why 
ts the same eclipse total at one place, and only partial at another f 



ECLIPSES. 


299 

898. Solar Eclipses. — When the Moon passes between 
the Earth and Sun, there happens an eclipse of the Sun, be - 
tan sc then the Moon's shadow falls upon the Earth. 

A total eclipse of the Sun happens often, but when it oc¬ 
curs the total obscurity is confined to a small part of the 
Earth; since the dark portion of the Moon’s shadow never 
exceeds 200 miles in diameter on the Earth. But the Moon’s 
partial shadow, or penumbra , may cover a space on the Earth 
of more than 4,000 miles in diameter, within alb which space 
the Sun will be more or less eclipsed. When the penumbra 
first touches the Earth, the< eclipse begins at that place, and 
ends when the penumbra leaves it. But the eclipse will be 
total only where the dark shadow of the Moon touches the 
Earth. * 


Fig. 225. 



Fig. 225 represents an eclipse of the Sun, without regard 
to the penumbra, that it may be observed how small a part 
of the Earth the dark shadow of the Moon covers. To those 
who live within the limits of this shadow, the eclipse will be 
total, while to those who live in any direction around it, and 
within reach of the penumbra, it will be only partial. 

899. Solar eclipses are called annular from annulus , a ring, 
when the Moon passes across the centre of the Sun, hiding 
all his light, with the exception of a ring on his outer edge, 
which the Moon - is too small to cover from the position in 
which it is seen. 

Umbra and Penumbra .—A solar eclipse, with the penum¬ 
bra, d , c, and the umbra , or dark shadow, is seen in fig. 226. 

When the Moon is at its greatest distance from the Earth, 
its shadow m o , sometimes terminates, before it reaches the 
Earth, and then an observer standing directly under the point 


Why is a total eclipse of the Sun confined to so small a part of the Earth ? 
What is meant by penumbra? What will be the difference in the aspect of the 
eclipse, whether the observer stands within the dark shadow, or only within 
the penumbra ? What is meant by annular eclipses ? Are annular eclipses 
ever total in any part of the Earth? In annular eclipses, what part of the 
Moon’s shadow reaches the Earth ? 



300 


TIDES. 


Fig. 226. 



o , will see the outer edge of the Sun, forming a bright ring 
around the circumference of the Moon, thus forming an an¬ 
nular eclipse. 

The penumbra d c, is only a partial interception of the 
Sun’s rays, and in annular eclipses it is this partial shadow 
only which reaches the Earth, while the umbra, or dark 
shadow, terminates in the air. Hence annular eclipses are 
never total in any part of the Earth. The penumbra, as al¬ 
ready stated, may cover more than 4,000 miles of space, while 
the umbra never covers more than 200 miles in diameter; 
hence partial eclipses of the Sun may be seen by a vast num 
ber of inhabitants, while comparatively few will witness the 
total eclipse. 

900. When there happens a total solar eclipse to us, we 
are eclipsed to the Moon, and when the Moon is eclipsed to 
us, an eclipse of the Sun happens to the Moon. To the 
Moon, an eclipse of the Earth can never be total, since her 
shadow covers only a small portion of the Earth’s surface. 
Such an eclipse, therefore, at the Moon, appears only as a dark 
spot on the face of the Earth; but when the Moon is eclipsed 
to us, the Sun is partially eclipsed to the Moon for several 
hours longer than the Moon is eclipsed to us. 

THE TIDES. 

901. The ebbing and flowing of the sea , which regularly 
takes place twice in 24 hours , are called the tides. The cause 
of the tides, is the attraction of the Sun and Moon, but chiefly 
of the Moon, on the waters of the ocean. In virtue of the 
universal principle of gravitation, heretofore explained, the 
Moon, by her attraction, draws, or raises the water towards 
her, but because the power of attraction diminishes as the 
squares of the distances increase, the waters, on the oppo¬ 
site side of the Earth, are not so much attracted as they are 

What is said concerning eclipses of the Earth, as seen from the Moon ? 
What are the tides ? What is the cause of the tides ? What causes the tide 
to rise on the side of the Earth opposite to the Moon ? 



TIDES. 


301 

on the side nearest the Moon. This want of attraction, to¬ 
gether with the greater centrifugal force of the Earth on its 
opposite side, produced in consequence of its greater distance 
trom the common centre of gravity, between the Earth and 
Moon, causes the waters to rise on the opposite side, at the 
same time that they are raised by direct attraction on the 
side nearest the Moon. 

Thus the waters are constantly elevated on the sides of the 
Earth opposite to each other above their common level, and 
consequently depressed at opposite points equally distant 
from these elevations. 

Let m, fig. 227, be the Moon, and E the Earth covered 


Fig. 227. 



with water. As the Moon passes round the Earth, its solid 
and fluid parts are equally attracted by her influence ac¬ 
cording to their densities; but while the solid parts are at 
liberty to move only as a whole, the water obeys the slight¬ 
est impulse, and thus tends towards the Moon where her at¬ 
traction is the strongest. Consequently, the waters are per¬ 
petually elevated immediately under the Moon. If, therefore, 
the Earth stood still, the influence of the Moon’s attraction 
would raise the tides only as she passed round the Earth. 
But as the Earth turns on her axis every 24 hours, and as 
the waters nearest the Moon, as at <z, are constantly elevated, 
they will, in the course of 24 hours, move round the whole 
Earth, and consequently from this cause there will be high 
water at every place once in 24 hours. As the elevation 
of the waters under the Moon causes their depression at 90 
degrees distance on the opposite sides of the Earth, d and c, 
the point c will come to the same place, by the Earth’s diur¬ 
nal revolution, six hours after the point a , because c is one 
quarter of the circumference of the Earth from the point a, 
and therefore there will be low water at any given place six 
hours after it was high water at that place. But while it is 


If the Earth stood still, the tides would rise only as the Moon passes round 
the Earth ; what then causes the tides to rise twice in 24 hour* ? 


26 




TIDES. 


30*2 

high water under the Moon, in consequence of her direct at¬ 
traction, it is also high water on the opposite side of the 
Earth in consequence of her diminished attraction, and the 
Earth’s centrifugal motion, and therefore it will be high wa¬ 
ter from this cause twelve hours after it was high water from 
the former cause, and six hours after it was low water from 
both causes. 

Thus, when it is high water at a and b , it is low water at 
c and d, and as the Earth revolves once in 24 hours, there 
will be an alternate ebbing and flowing of the tide, at every 
place, once in six hours. 

But while the Earth turns on her axis, the Moon advances 
in her orbit, and consequently any given point on the Earth 
will not come under the Moon on one day so soon as it did 
on the day before. For this reason, high or low water at any 
place comes about fifty minutes later on one day than it did 
the day before. 

Thus far we have considered no other attractive influence 
except that of the Moon, as affecting the waters of the ocean. 
But the Sun, as already observed, has an effect upon the 
tides, though on account of his great distance, his influence 
is small when compared with that of the Moon. 

902. When the Sun and Moon are in conjunction, as 
represented in fig. 227, which takes place at her change, or 
when they are in opposition, which takes place at full Moon, 
then their forces are united, or act on the waters in the same 
direction, and consequently the tides are elevated higher than 
usual, and on this account are called spring tides. 

903. But when the Moon is in her quadratures, or quar¬ 
ters. the attraction of the Sun tends to counteract that of the 
Moon, and although his attraction does not elevate the waters 
and produce tides, his influence diminishes that of the Moon, 
and consequently the elevation of the waters are less when 
the Sun and Moon are so situated in respect to each other, 
than when they are in conjunction, or opposition. 

This effect is represented by fig. 228, where the elevation 
of the tides at c and d is produced by the causes already 
explained ; but their elevation is not so great as in fig. 227, 
since the influence of the Sun acting in the direction a b , 
tends to counteract the Moon’s attractive influence. These 

When it is high water under the Moon by her attraction, what is the cause 
of high water on the opposite side of the Earth, at the same time? Why are 
the tides about fifty minutes later every day ? What produces spring tides ? 
Where must the Moon be in respect to the Sun, to produce spring tides r 
What is the occasion of neap tides f 




LATITUDE AND LONGITUDE. 


303 


small tides are called neap tides , and happen only when tho 
Moon is in her quadratures. 

Fig. 228. 


The tides are not at their greatest heights at the time 
when the Moon is at its meridian, but some time afterwards, 
because the water, having a motion forward, continues to 
advance by its own inertia, some time after the direct influ¬ 
ence of the Moon has ceased to affect it. 

LATITUDE AND LONGITUDE. 

904. Latitude is the distance from the equator in a direct 
line , north or south, measured in degrees and minutes. The 
number of degrees is 90 north, and as many south, each line 
on which these degrees are reckoned running from the equa¬ 
tor to the poles. Places at the north of the equator are in 
north latitude , and those south of the equator are in south lati¬ 
tude. The parallels of latitude are imaginary lines drawn 
parallel to the equator, either north or south, and hence 
every place situated on the same parallel, is in the same 
latitude because every such place must be at the same dis¬ 
tance from the equator. The length of a degree of latitude 
is 60 geographical miles. 

905. Longitude is the distance measured in degrees and min¬ 
utes, either east or west^from any given point on the equator , or 
on any parallel of latitude. Hence the lines, or meridians of 
longitude, cross those of latitude at right-angles. The de¬ 
grees of longitude are 180 in number, its lines extending half 
a circle to the east, and half a circle to the west, from any 
given meridian, so as to include the whole circumference of 
the Earth. A degree of longitude, at ihe equator, is of the 
same length as a degree of latitude, but as the poles are ap- 




What is latitude ? How many degrees of latitude are there ? How far do 
the lines of latitude extend ? What is meant by north and south latitude ? 
What are the parallels of latitude ? What is longitude T How many degrees 
of longitude are there, east or west ? 




304 


LATITUDE AND LONGITUDE. 


Fig. 229. 

N 



proached, the degrees of longitude diminish in length, be* 
cause the Earth grows smaller in circumference from the 
equator towards the poles ; hence the lines surrounding it be¬ 
come less and less. This will be made obvious by fig. 229. 

Let this figure represent the 
Earth, N being the north 
pole, S the south pole, and 
E W the equator. The lines 
10,20, 30, and so on, are the 
parallels of latitude, and the 
lines N a S, N b S, &c., are 
meridian lines, or those of 
longitude. 

The latitude of any place 
on the globe, is the number 
of degrees between that place 
and the equator, measured on 
a meridian line ; thus, x is in 
latitude 40 degrees, because 
the x g part of the meridian 
contains 40 degrees. 

The longitude of a place is the number of degrees it is 
situated east or west from any meridian line; thus, v is 20 
degrees west longitude from x 1 and x is 20 degrees east lon¬ 
gitude from v. 

906. As the equator divides the Earth into two equal parts, 
or hemispheres, there seems to be a natural reason why the 
degrees of latitude should be reckoned from this great circle. 
But from east to west there is no natural division of the 
Earth, each meridian line being a great circle, dividing the 
Earth into two hemispheres, and hence there is no natural 
reason why longitude should be reckoned from one meridian 
any more than another. It has, therefore, been customary for 
writers and mariners to reckon longitude from the capital of 
their own country ; as the English from London, the French 
from Paris, and the Americans from Washington. But this 
mode, it is apparent, must occasion much confusion, sinca 
each writer of a different nation would be obliged to correct 
the longitude of all other countries, to make it agree with his 
own. More recently, therefore, the writers of Europe and 
America have selected the royal observatory, at Greenwich, 


What is the latitude of any place ? What is the longitude of a place ? Why 
are the degrees of latitude reckoned from the equator ? What is said concern¬ 
ing the places from which the degrees of longitude have been reckoned ? 












LATITUDE AND LONGITUDE. 


305 

near London, as the first meridian, and on most maps and 
charts lately published, longitude is reckoned from that place. 

907. How Latitude is found .—The latitude of any place 
is determined by taking the altitude of the Sun at mid-day, 
and then subtracting this from 90 degrees, making proper 
allowances for the Sun’s place in the heavens. The reason 
of this will be understood, when it is considered that the 
whole number of degrees from the zenith to the horizon is 
90, and therefore if we ascertain the Sun’s distance from the 
horizon, that is, his altitude, by allowing for the Sun’s de¬ 
clination north or south of the equator, and subtracting this 
from the whole number, the latitude of the place will be 
found. Thus, suppose that on the 20th of March, when the 
Sun is at the equator, his altitude from any place north of the 
equator should be found to be 48 degrees above the horizon ; 
this, subtracted from 90, the whole number of the degrees of 
latitude, leaves 42, which will be the latitude of the place 
where the observation was made. 

908. If the Sun, at the time of observation, has a declina¬ 
tion north or south of the equator, this declination must be 
added to, or subtracted from, the meridian altitude, as the case 
may be. For instance, another observation being taken at 
the place where the latitude was found to be 42, when the 
Sun had a declination of 8 degrees north, then his altitude 
would be 8 degrees greater than before, and therefore 56, 
instead of 48. Now, subtracting this 8, the Sun’s declina¬ 
tion, from 56, and the remainder from 90, and the latitude of 
the place will be found 42, as before. If the Sun’s declina¬ 
tion be south of the equator, and the latitude of the place 
north, his declination must be added to the meridian altitude 
instead of being subtracted from it. The same result may 
be obtained by taking the meridian altitude of any of the fixed 
stars, whose declinations are known, instead of the Sun’s, 
and proceeding as above directed. 

909. How Longitude is found .—There is more difficulty 
in ascertaining the degrees of longitude, than those of lati¬ 
tude, because, as above stated, there is no fixed point, like 
that of the equator, from which its degrees are reckoned. 


What is the inconvenience of estimating longitude from a place in eacn 
country? From what place is longitude reckoned in Europe and America? 
How is the latitude of a place determined ? Give an example of the method 
of finding the latitude of the same place at different seasons of the year. When 
must the Sun’s declination from the equator be added to, and when subtracted 
from, his meridian altitude ? Why is there more difficulty in ascertaining the 
degrees of longitude than of latitude ? 

26* 



306 


LATITUDE AND LONGITUDE. 


Tlie degrees of longitude are therefore estimated from Green¬ 
wich, and are ascertained by the following methods:— 

910. When the Sun comes to the meridian of any place, 
it is noon, or 12 o’clock, at that place, and therefore, since 
the equator is divided into 360 equal parts, or degrees, and 
since the Earth turns on its axis once in 24 hours, 15 degrees 
of the equator will correspond with one hour of time, for 360 
degrees being divided by 24 hours, will give 15. The Earth, 
therefore, moves in her daily revolution, at the rate of 15 de¬ 
grees for every hour of time. Now, as the apparent course of 
the Sun is from east to west, it is obvious that he will come to 
any meridian lying east of a given place, sooner than to one 
lying west of that place, and therefore it will be 12 o’clock 
to the east of any place, sooner than at that place, or to the 
west of it. When, therefore, it is noon at any one place, it 
will be 1 o’clock at all places 15 degrees to the east of it, 
because the Sun was at the meridian of such places an hour 
before; and so, on the contrary, it will be eleven o’clock, 
fifteen degrees west of the same place, because the Sun has 
still an hour to travel before he reaches the meridian of that 
place. It makes no difference, then, where the observer is 
placed, since, if it is 12 o’clock where he is, it will be 1 o’clock 
15 degrees to the east of him, and 11 o!clock 15 degrees to 
the west of him, and so in this proportion, let the time be 
more or less. Now, if any celestial phenomenon should hap¬ 
pen, such as an eclipse of the Moon, or of Jupiter’s satellites, 
the difference of longitude between two places where it is 
observed, may be determined by the difference of the times 
at which it appeared to take place. Thus, if the Moon 
enters the Earth’s shadow at 6 o’clock in the evening, as 
seen at Philadelphia, and at half past 6 o’clock at another 
place, then this place is half an hour, or 7\ degrees, to the 
east of Philadelphia, because degrees of longitude are 
equal to half an hour of time. To apply these observations 
practically, it is only necessary that it should be known ex¬ 
actly at what time the eclipse takes place at a given point on 
the Earth. 

911. Use of the Chronometer. —Longitude is also ascer¬ 
tained by means of a chronometer, or true time piece, adjusted 
to any given meridian; for if the difference between two 

How many degrees of longitude does the surface of the Earth pass through 
m an hour ? Suppose it is noon at any given place, what o’clock will it he 15 
degrees to the east of that place ? Explain the reason. How r may longitude 
be determined by an eclipse? Explain the principles on which longitude a 
determined by the chronometer. 



FIXED STARS. 


307 


clocks situated east and west of each other, and going ex¬ 
actly at the same rate, can be known at the same time, then 
the distance between the two meridians, where the clocks 
are placed, will be known, and the difference of longitude 
may be found. 

Suppose two chronometers, which are known to go at ex¬ 
actly the same rate, are made to indicate 12 o’clock by the 
meridian line at Greenwich, and the one be taken to sea, 
while the other remains at Greenwich. Then suppose the 
captain, who takes his chronometer to sea, has occasion to 
know his longitude. In the-first place, he ascertains, by an 
observation of the Sun, when it is 12 o’clock at the place 
where he is, and then by his time piece, when it is 12 o’clock 
at Greenwich, and by allowing 15 degrees for every hour of 
the difference in time, he will know his precise longitude in 
any part of the world. For example, suppose the captain 
sails with his chronometer for America, and after being sev¬ 
eral weeks at sea, finds by observation that it is 12 o’clock 
by the Sun, and at the same time finds by his chronometer, 
that it is 4 o’clock at Greenwich. Then because it is noon 
at his place of observation after it is noon at Greenwich, he 
knows that his longitude is west from Greenwich, and by al¬ 
lowing 15 degrees for every hour of the difference, his lon¬ 
gitude is ascertained. Thus, 15 degrees, multiplied by 4 
hours, give 60 degrees of west longitude from Greenwich. 
If it is noon at the place of observation, before it is noon at 
Greenwich, then the captain knows that his longitude is east, 
and his true place is found in the same manner. 

FIXED STARS. 

912. The stars are called fixed , because they have been ob¬ 
served not to change their places with respect to each other. 
They may be distinguished by the naked eye from the plan¬ 
ets of our system by their scintillations, or twinkling. The 
stars are divided into classes, according to their magnitudes, 
and are called stars of the first, - second, and so on to the 
sixth magnitude. About 2,000 stars may be seen with the 
naked eye in the whole vault of the heavens, though only 
about 1,000 are above the horizon at the same time. Of these, 

Suppose the captain finds by his chronometer that it is 12 o’clock, where ht 
is, six hours later than at Greenwich, what then would be his longitude ? Sup 
pose he finds it to be 12 o’clock 4 hours earlier, where he is, than at Green 
wich, what then would be his longitude ? Why are the stars called fixed . 
IIow may the stars be distinguished from the planets ? The stars are divided 
into classes, according to their magnitudes ; how many classes are there ? 


* 




308 


FIXED STARS. 


about 17 aieof the first magnitude, 50 of the 2d magnitude, 
ai.d 150 of the 3d magnitude. The others are of the 4th, 
5th, and 6th magnitudes, the last of which are the smallest 
that can be distinguished with the naked eye. 

913. It might seem incredible, that on a clear night only 
about 1,000 stars are visible, when on a single glance at the 
different parts of the firmament, their numbers appear innu¬ 
merable. But this deception arises from the confused and 
hasty manner in which they are viewed, for if we look stea¬ 
dily on a particular portion of sky, and count the stars con¬ 
tained within certain limits, we shall be surprised to find their 
number so few. 

914. As we have incomparably more light from the 
Moon than from all the stars together, it is absurd to suppose 
that they were made for no other purpose than to cast so faint 
a glimmering on our Earth, and especially as a great propor¬ 
tion of them are invisible to our naked eyes. The nearest 
fixed stars to our system, from the most accurate astronomi¬ 
cal calculations, cannot be nearer than 20,000,000,000,000, 
or 20 trillions of miles from the Earth, a distance so immense, 
that light cannot pass through it in less than three years. 
Hence, were these stars annihilated at the present time, their 
light would continue to flow towards us, and they would ap¬ 
pear to be in the same situation to us, three years hence, that 
they do now. 

915. Our Sun, seen from the distance of the nearest fixed 
stars, would appear no larger than a star of the first magni¬ 
tude does to us. These stars appear no larger to us, when 
the Earth is in that part of her orbit nearest to them, than 
they do, when she is in the opposite part of her orbit; and as 
our distance from the Sun is 95,000,000 of miles, we must 
be twice this distance, or the whole diameter of the Earth’s 
orbit, nearer a given fixed star at one period of the year than 
at another. The difference, therefore, of 190,000,000 of 
miles, bears so small a proportion to the whole distance be¬ 
tween us and the fixed stars, as to make no appreciable dif¬ 
ference in their sizes, even when assisted by the most power¬ 
ful telescopes. 


How many stars may be seen with the naked eye, in the whole firmament ? 
Why does there appear to be more stars than there really are ? What is the 
computed distance of the nearest fixed stars from the Earth ? How long would 
it take light to reach us from the fixed stars ? How large would our Sun ap¬ 
pear at the distance of the fixed stars ? What is said concerning the differ¬ 
ence of the distance between the Earth and the fixed stars at different season* 
of the year, and of their different appearance in consequence ? 




PLANETARIUM. 


309 


916. The amazing distances of the fixed stars may also be 
inferred from the return of comets to our system, after an 
absence of several hundred years. 

The velocity with which some of these bodies move, when 
nearest the Sun, has been computed at nearly a million of 
miles in an hour, and although their velocities must be per¬ 
petually retarded, as they recede from the Sun, still, in 250 
years of time, they must move through a space which to us 
would be infinite. The periodical return of one comet is 
known to be upwards of 500 years, making more than 250 
years in performing its journey to the most remote part of its 
orbit, and as many in returning back to our system; and 
that it must still always be nearer our system than the fixed 
stars, is proved by its return ; for by the laws of gravitation, 
did it approach nearer another system it would never again 
return to ours. 

From such proofs of the vast distances of the fixed stars, 
there can be no doubt that they shine with their own light, 
like our Sun, and hence the conclusion that they are suns to 
other worlds, which move around them, as the planets do 
around our Sun. Their distances will, however, prevent our 
ever knowing, except by conjecture, whether this is the case 
or not, since, were they millions of times nearer us than they 
are, we should not be able to discover the reflected light of 
their planets. 


PLANETARIUM. 

917. The author is under lasting obligations to Mr. Haz- 
well, the proprietor of Russell’s Planetarium, for the follow¬ 
ing stereotype cut, and description of that wonderful instru¬ 
ment, both of which he was so kind as to present him for 
publication in this work. 

Explanation .—The numbers on the cut have the following 
references, No. l,the Sun; 2, Mercury; 3, Venus; 4, Earth; 
5, Mars; 6, Asteroids; 7, Jupiter; 8, Saturn; 9, Herschel. 

918. Russell’s Planetarium is, as the term implies, a work¬ 
ing model of the solar system. It comprehends all the 
bodies known up to the present time to belong to that sys¬ 
tem, except the comets. It includes the new planets, and 
all the satellites. The dimensions of this stupendous piece 
of mechanism may be conceived, when it is stated that the 


How may the distances of the fixed stars be inferred, by the long absence 
and return of comets ? On what grounds is it supposed that the fixed stars 
are suns to other worlds f 



310 


PLANETARIUM. 


Fis. 230. 



extreme planet, Kerschel, moves round the sun in a circum¬ 
ference which measures from seventy to eighty feet. The 
Sun, standing in the centre, is a ground glass globe, con¬ 
taining a light within it, reflecting, as in nature, its beams in 
every direction around and illuminating the circumvolving 
planets. On the Sun the spots are represented, and it re¬ 
volves on its axis, carrying its spots with it. Contiguous to 
the Sun the planet Mercury appears, moving in his orbit, and 
showing at the same time his diurnal motion of rotation, and 
the obliquity of his axis. Venus, represented by a silver ball, 
next succeeds, moving in her proper time round the Sun, and 
spinning on her proper axis. Beyond Venus the Earth re¬ 
volves, accompanied by the Moon. To the Earth and Moon 
all the motions are simultaneously imparted by a simple and 
beautiful combination of mechanical expedients. Like the 
other planets, it has its annual and diurnal motions. Its 
axis is properly inclined, showing the succession of seasons, 
and the inequality of the days and nights. But this is not all. 
By an arrangement of a simple and beautiful kind, it re¬ 
ceives its elliptical motions, showing its aphelion and peri¬ 
helion. The motions of the Moon are given with no less 
scrupulous precision. Its monthly course, its motion on its 
axis, the obliquity of its orbit, the position, and even the mo¬ 
tion of its nodes, are all faithfully executed. 

919. The Earth and minor planets are surrounded by a mag¬ 
nificent armillary sphere, (10) the circles of which are formed 
of polished brass and steel, and which measures fifteen feet 
in circumference. This sphere consists of the great meridian, 









PLANETARIUM. 


311 


the colures, the celestial equator and its parallels. Ini' 
mediately outside the sphere the planet Mars revolves in his 
proper time, showing at the same time his diurnal motion 
and the geographical character of his surface, the obliquity 
of his orbit being also observed. Beyond Mars and at nearly 
equal distances, revolves the four newly discovered planets, 
Pallas, Ceres, Vesta, and Juno. The great obliquity of the 
orbits of some of these is exhibited, and the artist has gone 
before discovery, anticipating future observations, by giving 
them severally diurnal motions. 

920. We now encounter a wide unoccupied space, beyond 
which the magnificent system of Jupiter, attended by his 
four moons, revolve, with their several complicated motions. 
The planet, a noble object, appears surrounded by his belts, 
and rapidly spinning on his axis. His several moons move 
around him with the proper obliquities, dispensing floods of 
subsidiarjr light, and compensating for the distant and di¬ 
minished Sun. Further still, wheeling around in a circum¬ 
ference which measures above fifty feet, the majestic system 
of Saturn, his rings and satellites, appears. The planet shows 
its rapid diurnal motion on its obliquely-directed axis, display¬ 
ing the same inequality of days and nights and the same 
succession of seasons as prevail upon the Earth. Even the 
rings show the revolving motion discovered in them by Sir 
William Herschel. The gorgeous cortege of seven moons 
circulate beyond these rings, each having its proper motion 
and obliquity, and showing how they minister uninterrupted 
moonlight to the planet. Lastly, and at the extreme verge 
of the system, moves, in solemn slowness, the family of 
globes, of which the planet Herschel is the physical centre, 
presenting the anomalous spectacle of six moons moving 
perpendicularly to the common plane of the system, and in 
the immensity of their distance seeming to abandon, in that 
respect, the harmony and order which presides throughout it! 

921. The apparatus receives its various and complicated 
motions from a system of mechanism which is placed partly 
on the slender stems which sustain the planets and satellites, 
but chiefly on a richly decorated table, more than fifty feet in 
circumference, on which the whole apparatus is supported, 
and which stands itself on a massive pedestal of splendidly 
carved and fluted metal. The extreme height of the appa¬ 
ratus above the floor on which the pedestal rests is about 
twelve feet, but that of the principal planets does not exceed 
nine feet. 


312 


COMETS. 


The table is chiefly composed of polished metal, enamelled 
pieces, inlaid emblematical figures, the signs of the zodiac, 
the points of the compass, and beautifully engraved repre¬ 
sentations of the Moon, planets, comets, and other celestial 
objects. A handsomely painted carpet covers the floor on 
which the instrument stands, on which are delineated the 
twelve signs of the zodiac. 

Some faint idea may be formed of this fine piece of me 
chanism when it is known that its weight is about two tons, 
and its estimated value above ten thousand dollars. 

It would be unjust to close this brief notice without offering 
a tribute to the humble and unobtrusive genius to whom the 
world is indebted for the most splendid gift which art has 
presented to public instruction. 

922. Mr. James Russell, the sole inventor and constructor 
of this apparatus, is a native of New England, who has 
been for many years a resident of Columbus, Ohio. Sup¬ 
ported and aided in pecuniary means by a number of liberal 
gentlemen of that city, he has, after years of mental and 
bodily toil, succeeded in producing this unparalleled piece of 
illustrative mechanism, which no city of the old world can 
offer any similar object to equal. Nay, we are not overstep¬ 
ping the bounds of strict truth in saying that nothing of the 
kind which Europe has produced would even for a moment 
bear to be placed beside it. 

COMETS. 

923. Besides the planets, which move round the Sun in 
regular order and in nearly circular orbits, there belongs to 
the solar system an unknown number of bodies called Com - 
els , which move round the Sun in orbits exceedingly eccen¬ 
tric, or elliptical, and whose appearance among our heavenly 
bodies is only occasional. Comets, to the naked eye, have 
no visible disc, but shine with a faint, glimmering light, and 
are accompanied by a train or tail, turned from the Sun, and 
which is sometimes of immense length. They appear in 
every region of the heavens, and move in every possible-di¬ 
rection. 

In the days of ignorance and superstition, comets were 
considered the harbingers of war, pestilence, or some other 
great or general evil; and it was not until astronomy had 
made considerable progress as a science, that these strangers 
could be seen among our planets without the expectation of 
some direful event. 

924. It had been supposed that comets moved in straight 


(XJMETS. 


ni3 

iines, coming from the regions of infinite, or unknown space, 
and merely passing by our system, on their way to regions 
equally unknown and infinite, and from which they never 
returned. Sir Isaac Newton was the first to demonstrate 
that the comets pass round the Sun, like the planets, but that 
their orbits are exceedingly elliptical, and extend out to a 
vast distance beyond the solar system. 

925. The number of comets is unknown, though some as¬ 
tronomers suppose that there are nearly 500 belongingto our 
system. Ferguson, who wrpte in about 1760, supposed that 
there were less than 30 comets which made us occasional 
visits ; but since that period the elements of the orbits of nearly 
100 of these bodies have been computed. 

Of these, however, there are only three whose periods of 
return among us are known with any degree of certainty. 
The first of these has a 

period of 75 years ; the Fig. 231. 

second a period of 129 
years ; and the third a 
period of 575 years. 

The third appeared in 
1680, and therefore can¬ 
not be expected again 
until the year 2225. 

This comet, fig. 231. in 
1680, excited the most intense interest among the astrono¬ 
mers of Europe, on account of its great apparent size and 
near approach to our system. In the most remote part of its 
orbit, its distance from the Sun was estimated at about elev 
en thousand two hundred millions of miles. At its nearest 
approach to the Sun, which was only about 50,000 miles, 
its velocity, according to Sir Isaac Newton, was 880,000 
miles in an hour; and supposing it to have retained the 
Sun’s heat, like other solid bodies, its temperature must have 
been ajDOut 2000 times that of red hot iron. The tail of this 
comet was at least 100 millions of miles long. 

926. In the Edinburgh Encyclopedia, article Astronomy , 
there is the most complete table of comets yet published. 
This table contains the elements of 97 comets, calculated by 
different astronomers, down to the year 1808. 

From this table it appears that 24 comets have passed be- 

What number of comets are supposed to belong to our system ? How many 
have had the elements of their orbits estimated by astrohomers ? How many 
are there whose periods of return are known ? What is said of the comet 
of 1680? 



27 




































314 


ELECTRICITY. 


tween the Sun and the orbit of Mercury; 33 between the 
orbits of Venus and the Earth ; 15 between the orbits of the 
Earth and Mars ; 3 between the orbits of Mars and Ceres ; 
and 1 between the orbits of Ceres and Jupiter. It also ap¬ 
pears by this table that 49 comets have moved round the 
Sun from west to east, and 48 from east to west. 

927. Of the nature of these wandering planets very little 
is known. When examined by a telescope, they appear like 
a mass of vapors surrounding a dark nucleus. When the 
comet is at its perihelion, or nearest the Sun, its color seems 
to be heightened by the intense light or heat of that luminary, 
and it then often shines with more brilliancy than the planets. 
At this time the tail or train, which is always directly oppo¬ 
site to the Sun, appears at its greatest length, but is com¬ 
monly so transparent as to permit the fixed stars to be seen 
through it. A variety of opinions have been advanced by 
astronomers concerning the nature and causes of these trains. 
Newton supposed that they were thin vapor, made to as¬ 
cend by the Sun’s heat, as the smoke of a fire ascends from 
the earth; while Kepler maintained that it was the atmos¬ 
phere of the comet driven behind it by the impulse of the 
Sun’s rays. Others suppose that this appearance arises from 
streams of electric matter passing away from the comet, &c. 


ELECTRICITY. 

928. The science of Electricity , which now ranks as an 
important branch of Natural Philosophy, is wholly of modern 
date. The ancients were acquainted with a few detached 
facts dependent on the agency of electrical influence, but they 
never imagined that it was extensively concerned in the ope¬ 
rations of nature, or that it pervaded material substances gen¬ 
erally. The term electricity is derived from electron , the 
Greek name of amber, because it was known to the ancients, 
that when that substance was rubbed or excited, it attracted 
or repelled small light bodies, and it was then unknown that 
other substances when excited would do the same. 

929. When a piece of glass, sealing wax, or amber, is 
rubbed with a dry hand, and held towards small and light, 
bodies, such as threads, hairs, feathers, or straws, these 
bodies will fly towards the surface thus rubbed, and adhere 
to it for a short time. The influence by which these small 


From what is the term electricity derived ? What is electrical attraction ? 





ELECTRICITY. 


315 


substances are drawn, is called electrical attraction; the sur¬ 
face having this attractive power is said to be excited; and 
the substances susceptible of this excitation, are called elec - 
tries. Substances not having this attractive power when 
rubbed, are called non-electrics. 

930. The principal electrics are amber, rosin, sulphur, 
glass, the precious stones, sealing wax, and the fur of quad¬ 
rupeds. But the metals, and many other bodies, may be ex¬ 
cited when insulated and treated in a certain manner. 

After the light substances which had been attracted by the 
excited surface, have remained in contact with it a short 
time, the force which brought them together ceases to act, 
or acts in a contrary direction, and the light bodies are re- 
pelled , or thrown away from the excited surface. Two bodies, 
also, which have been in contact with the excited surface, 
mutually repel each other. 

931. Various modes have been devised for exhibiting dis¬ 
tinctly the attractive and repulsive agencies of electricity, and 
for obtaining indications of its presence, when it exists only 
in a feeble degree. Instruments for this purpose are termed 
Electroscopes. 

932. One of the simplest instruments of this kind consists 
of a metallic needle, terminated at each end by a light pith 
ball, which is covered with gold leaf, and supported horizon¬ 
tally at its centre by a fine point, fig. 232. When a stick of 
sealing wax, or a glass tube, is 
excited, and then presented to 
one of these balls, the motion of 
the needle on its pivot will indi¬ 
cate the electrical influence. 

933. If an excited substance 
be brought near a ball made of 
pith, or cork, suspended by a 
silk thread, the ball will, in the 
first place, approach the electric, 
as at a, fig. 333, indicating an 
attraction towards it, and if the 
position of the electric will al¬ 
low, the ball will come into con¬ 
tact with the electric, and ad¬ 
here to it for a short time, and 
will then recede from it, show- 


Fig. 232. 

o - a o 


CiB> 

Fig. 333. 



What are electrics ? What are non-electrics T What are the principal elec¬ 
trics ? What is meant by electrical repulsion ? What is an electroscope T 











ELECTRICITY. 


316 

ing that it is repelled, as at b. If now the ball which had 
touched the electric, be brought near another ball, which has 
had no communication with an excited substance, these two 
balls will attract each other, and come into contact; after 
which they will repel each other, as in the former case. 

934. It appears, therefore, that the excited body, as the 
stick of sealing wax, imparts a portion of its electricity to the 
ball, and that when the ball is also electrified, a mutual re¬ 
pulsion then takes place between thefii. Afterwards, the 
ball, being electrified by contact with the electric, when 
brought near another ball not electrified, transfers a part of 
its electrical influence to that, after which these two balls 
repel each other, as in the former instance. 

935. Thus, when one substance has a greater or less quan¬ 
tity of electricity than another, it will attract the other sub¬ 
stance, and when they are in contact will impart to it a por¬ 
tion of this superabundance; but when they are both equally 
electrified, both having more or less than their natural quan¬ 
tity of electricity, they will repel each other. 

936. To account for these phenomena, two theories have 
been advanced, one by Dr. Franklin, who supposes there is 
only one electrical fluid, and the other by Du Fay, who sup¬ 
poses that there are two distinct fluids. 

937. Dr. Franklin supposed that all terrestrial substances 
were pervaded with the electrical fluid, and that by exciting 
an electric, the equilibrium of this fluid was destroyed, so 
that one part of the excited body contained more than its 
natural quantity of eclectricity, and the other part less. If in 
this state a conductor of electricity, as a piece of metal, be 
brought near the excited part, the accumulated electricity 
would be imparted to it, and then this conductor would re¬ 
ceive more than its natural quantity of the electric fluid. 
This he called positive electricity. But if a conductor be 
connected with that part which has less than its ordinary 
share of the fluid, then the conductor parts with a share of 
its own, and therefore will then contain less than its natural 
quantity. This he called negative electricity. When orre 
body positively and another negatively electrified, are con¬ 
nected by a conducting substance, the fluid rushes from the 


When do two electrified bodies attract, and when do they repel each other? 
How will two bodies act, one having more, and the other less, than the natural 
quantity of electricity, when brought near each other ? How will they act when 
both have more or less than their natural quantity? Explain Dr. Franklin’s 
theory of electricity. What is meant by positive, and what by negative elec¬ 
tricity ? 



F.LECTRIOITV. 


317 

positive to the negative body, and the equilibrium is restored. 
Thus, bodies which are said to be positively electrified, con¬ 
tain more than their natural quantity of electricity, while 
those which are negatively electrified contain less than their 
natural quantity. 

938. The other theory is explained thus. When a piece 
of glass is excited and made to touch a pith ball, as above 
stated, then that ball will attract another ball, after which 
they will mutually repel each other, and the same will hap¬ 
pen if a piece of sealing-wax be used instead of the glass. 
But if a piece of excited glass, and another of wax, be made 
to touch two separate balls, they will attract each other; 
that is, the ball which received its electricity from the wax 
will attract that which received its electricity from the glass, 
and will be attracted by it. Hence Du Fay concludes that 
electricity consists of two distinct fluids, which exist together 
in all bodies—that they have a mutual attraction for each 
other—that they are separated by the excitation of electrics, 
and that when thus separated, and transferred to non-elec¬ 
trics, as to the pith balls, their mutual attraction causes 
the balls to rush towards each other. These two principles 
he called vitreous and resinous electricity. The vitreous 
being obtained from glass, and the resinous from wax and 
other resinous substances. 

939. Dr. Franklin’s theory is by far the most simple, and 
will account for most of the electrical phenomena equally 
well with that of Du Fay, and therefore has been adopted by 
the most able and recent electricians. 

940. It is found that some substances conduct the electric 
fluid from a positive to a negative surface with great facility, 
while others conduct it with difficulty, and others not at all. 
Substances of the first kind are called conductors , and those 
of the last non-conductors. The electrics, or such sub¬ 
stances as being excited communicate electricity, are all 
non-conductors, while the non-electrics, or such substan¬ 
ces as do not communicate electricity on being merely ex¬ 
cited, are conductors. The conductors are the metals, char¬ 
coal, water, and other fluids, except the oils; also smoke, 


What is the consequence, when a positive and a negative body are connected 
by a conductor? Explain Du Fay’s theory. When two balls are electrified, 
one with glass, and the other with wax, will they attract or repel each other ? 
What are the two electricities called ? From what substances are the two 
electricities obtained ? What are conductors ? What are non-conductors ? 
What substances are conductors ? 


27’ 




ELECTRICITY. 


M8 

steam, ice, and snow. The best conductors are gold, silver, 
platina, brass, and iron. 

The electrics, or non-conductors, are glass, amber, sulphur, 
resin, wax, silk, most hard stones, and the furs of some ani¬ 
mals. 

941. A body is said to be insulated , when it is supported 
or surrounded by an electric. Thus, a stool standing on glass 
legs, is insulated, and a plate of metal laid on a plate of 
glass, is insulated. 

942. When large quantities of the electric fluid are wanted 
for experiment, or for other purposes, it is procured by an 
electrical machine. These machines are of various forms, but 
all consist of an electric substance of considerable dimen¬ 
sions ; the rubber by which this is excited ; the prime conduc¬ 
tor , on which the electric matter is accumulated ; the insula¬ 
tor , which prevents the fluid from escaping; and machinery, 
by which the electric is set in motion. 


Fig. 233. 



943. Fig. 233 represents such a machine, of which A is 
the electric, being a cylinder of glass ; B the prime conduc¬ 
tor ; R the rubber or cushion, and C a chain connecting the 
rubber with the ground. The prime conductor is supported 
by a standard of glass. Sometimes, also, the pillars which 
support the axis of the cylinder, and that to which the cush- 

What substances are the best conductors ? What substances are electrics, or 
non-conductors ? When is a body said to be insulated ? What are the ser 
eral parts of an electrical machine ? 
























ELEC rill CITY. 


319 

ion is attached, are made of the same material. The prime 
conductor has several wires inserted into its side, or end 
which are pointed, and stand with the points near the cylin¬ 
der. They receive the electric fluid from the glass, and con¬ 
vey it to the conductor. The conductor is commonly made 
of sheet brass, there being no advantage in having it solid, as 
the electric fluid is always confined entirely to the surface. 
Even paper, covered with gold leaf, is as effective in this re¬ 
spect, as though the whole was of solid gold. The cushion 
is attached to a standard, which is furnished with a thumb 
screw, so that its pressure on the cylinder can be increased 
or diminished. The cushion is made of leather, stuffed, and 
at its upper edge there is attached a flap of silk, F, by which 
a greater surface of the glass is covered, and the electric fluid 
thus prevented, in some degree, from escaping. The efficacy 
of the rubber in producing the electric excitation is much in¬ 
creased by spreading on it. a small quantity of an amalgam 
of tin and mercury, mixed with a little lard, or other unctuous 
substance. 

944. The manner in which this machine acts, may be in¬ 
ferred from what has already been said, for when a stick of 
sealing-wax, or a glass tube, is rubbed with the hand, or a 
piece of silk, the electric fluid is accumulated on the excited 
substance, and therefore must be transferred from the hand, 
or silk, to the electric. In the same manner, when the cyl¬ 
inder is made to revolve, the electric matter, in consequence 
of the friction, leaves the cushion, and is accumulated on the 
glass cylinder, that is, the cushion becomes negatively, and 
the glass positively electrified. The fluid, being thus exci¬ 
ted, is prevented from escaping by the silk flap, until it 
comes to the vicinity of the metallic points, by which it is 
conveyed to the prime conductor. But if the cushion is 
insulated, the quantity of electricity obtained will soon have 
reached its limit, for when its natural quantity has been 
transferred to the glass, no more can be obtained. It is then 
necessary to make the cushion communicate with the ground, 
which is done by laying the chain on the floor, or table, 
when more of the fluid will be accumulated, by further 


What is the use of the pointed wires in the prime conductor ? How is it 
accounted for, that a mere surface of metal will contain as much electric fluid 
as though it were solid ? When a piece of glass, or sealing-wax, is excited, 
by rubbing it with the hand, or a piece of silk, whence comes the electricity ? 
When the cushion is insulated, why is there a limited quantity of electric mat¬ 
ter to be obtained from it ? What is then necessary, that more electric matter 
may be obtained from the cushion T 



320 


ELECTRICITY. 


excitation, the ground being the inexhaustible source of the 
electric fluid. 

945. If a person who is insulated takes the chain in his 
hand, the electric fluid will be drawn from him, along the 
chain, to the cushion, and from the cushion will be transferred 
to the prime conductor, and thus the person will become 
negatively electrified. If, then, another person, standing on 
the floor, hold his knuckle near him who is insulated, a 
spark of electric fire will pass between them, with a crack¬ 
ling noise, and the equilibrium will be restored; that is, the 
electric fluid will pass from him who stands on the floor, to 
him who stands on the stool. But if the insulated person 
takes hold of a chain, connected with the prime conductor, 
he may be considered as forming a part of the conductor, and 
therefore the electric fluid will be accumulated all over his 
surface, and he will be positively electrified, or will obtain 
more than his natural quantity of electricity. If now a per¬ 
son standing on the floor touch this person, he will receive a 
spark of electrical fire from him, and the equilibrium will 
again be restored. 

946. If two persons stand on two insulated stools, or if 
they both stand on a plate of glass, or a cake of wax, the 
one person being connected by the chain with the prime con¬ 
ductor, and the other with the cushion, then, after working 
the machine, if they touch each other, a much stronger 
shock will be felt than in either of the other cases, because 
the difference between their electrical states will be greater, 
the one having more and the other less than his natural 
quantity of electricity. But if the two insulated persons both 
take hold of the chain connected with the prime conductor, 
or with that connected with the cushion, no spark will pass 
between them, on touching each other, because they will 
then both be in the same electrical state. 

947. We have seen, fig. 232, that the pith ball is first 
attracted and then repelled, by the excited electric, and that 
the ball so repelled will attract, or be attracted by other sub¬ 
stances in its vicinity, in consequence of having received 


If an insulated person takes the chain, connected with the cushion, in his 
hand, what change will be produced in his natural quantity of electricity ? If 
the insulated person takes hold of the chain connected with the prime conduc¬ 
tor, and the machine be worked, what then will be the change produced in his 
electrical state ? If two insulated persons take hold of the two chains, one 
connected with the prime conductor/and the other with the cushion, what 
changes will be produced ? If they both take hold of the same chain, what 
will be the effect? 



ELECTRICITY. 


321 


from the excited body more than its ordinary quantity of 
electricity. 

These alternate movements are F - 
amusingly exhibited, jsy placing some lg * ’ ® 
small light bodies, such as the figures 
of men and women, made of pith, or 
paper, between two metallic plates, 
the one placed over the other, as in 
fig 1 . 234, the upper plate communica¬ 
ting with the prime conductor, and 
the other with the ground. When 
the electricity is communicated to the 
upper plate, the little figures, being 
attracted by the electricity, will jump 
up and strike their heads against it, 
and having received a portion of the 
fluid, are instantly repelled, and again 
attracted by the lower plate, to which 
they impart their electricity, and then are again attracted, 
and so fetch and carry the electric fluid from one to the 
other, as long as the upper plate contains more than the 
lower one. In the same manner, a tumbler, if electrified on 
the inside, and placed over light substances, as pith balls, 
will cause them to dance for a considerable time. 

948. This alternate attraction and repulsion, by movable 
conductors, is also pleasingly illustrated with a ball, sus¬ 
pended . by a silk string between 
two bells of brass, fig. 235, one 
of the bells being electrified, and 
the other communicating with the 


Fig. 235. 



ground. The alternate attraction 
and repulsion, moves the ball from 
one bell to the other, and thus pro¬ 
duces a continual ringing. In all 
these cases, the phenomena will 
be the same, whether the elec> 
tricity be positive or negative ; for 
two bodies, being both positively 
or negatively electrified, repel each 
other, but if one be electrified posi- 


ho a 


Explain the reason why the little images dance between the two metallic 
plates, fig. 234. Explain fig. 235. Does it make any difference in respect to 
the motion of the images, or of the ball between the bells, whether the eiee 
tricity be positive or negative 7 










8 22 


ELECTRICITY. 


tively, and the other negatively, or not at all, they attract 
each other. 

Thus, a small figure, in the human shape, with the head 
covered with hair, when electrified, either positively or nega¬ 
tively, will exhibit an appearance of the utmost terror, each 
hair standing erect, and diverging from the other, in conse¬ 
quence of mutual repulsion. A person standing on an insu¬ 
lated stool, and highly electrified, will exhibit the same 
appearance. In cold, dry weather, the friction produced 
by combing a person’s hair, will cause a less degree of the 
same effect. In either case, the hair will collapse, or shrink 
to its natural state, on carrying a needle near it, because this 
conducts away the electric fluid. Instruments designed 
to measure the intensity of electric action, are called elec¬ 
trometers. 

949. Such an instrument is represented by fig. 236. It 
consists of a slender rod of light wood, a, terminated by a 
pith ball, which serves as an index. This is suspended at 
the upper part of the wooden stem b , so as to play easily 
backwards and forwards. Ths ivory semicircle c, is affixed 
to the stem, having its centre coinciding with the axis of 
motion of the rod, so as to measure the angle of deviation 
from the perpendicular, which the repulsion of the ball from 
the stem produces on the index. 

When this instrument is used, the lower 
end of the stem is set into an aperture in the 
prime conductor, and the intensity of the 
electric action is indicated by the number of 
degrees the index is repelled from the perpen¬ 
dicular. 

The passage of the electric fluid through 
a perfect conductor is never attended with 
light, or the crackling noise which is heard 
when it is transmitted through the air, or 
along the surface of an electric. 

950. Several curious experiments illustrate 
this principle, for if fragments of tin foil, or 
other metal, be pasted on a piece of glass, so 

near each other that the electric fluid can pass between 
them, the whole line thus formed with the pieces of metal, 


Fiz. 236. 



When a person is highly electrified, why does he exhibit, an appearance ot 
the utmost terror ? What is an electrometer ? Describe that represented in 
fig. 236, together with the mode of using it. When the electric fluid passes 
along a perfect conductor, is it attended with light and noise, or not 7 






ELECTRICITY. 


323 


will be illuminated by the passage of the electricity from one 
to the othe ( r. 

Fig. 237 




. 0*2 


O 0 


o c Q c ® o 
' ? o” 


,o°o 

O O o 

° 0 o°o 

oO° OC 1 


951. In this manner, figures or words may be formed, as 
in fig. 237, which, by connecting one of its ends with the 
prime conductor, and the other with the ground, will, when 
the electric fluid is passed through the whole, in the dark, 
appear one continuous and vivid line of fire. 

952. Electrical light seems not to differ, in any respect, 
from the light of the Sun, or of a burning lamp. Dr. Wol- 
laston observed, that when this light was seen through a 
prism, the ordinary colors arising from the decomposition of 
light were obvious. 

953. The brilliancy of electrical sparks is proportional to 
the conducting power of the bodies between which it passes. 
When an imperfect conductor j such as a piece of wood, is 
employed, the electric light appears in faint, red streams, 
while, if passed between two pointed metals, its color is of a 
more brilliant red. Its color also differs, according to the 
kind of substance from, or to which, it passes, or it is depend¬ 
ent on peculiar circumstances. Thus, if the electric fluid 
passes between two polished metallic surfaces, its color is 
nearly white; but if the spark is received by the finger from 
such a surface, it will be violet. The sparks are green , when 
taken by the finger from a surface of silvered leather; yellow , 
when taken from finely powdered charcoal; and purple , 
when taken from the greater number of imperfect conductors. 

954. When the electric fluid is discharged from a point, ^ 
it is always accompanied by a current of air, whether the 
electricity be positive or negative. The reason of this appears 
to be, that the instant a particle of air becomes electrified, it 
repels, and is repelled, by the point from which it received 
the electricity. 


When it passes along an electric, or through the air, what phenomena does 
it exhibit ? Describe the experiment, fig. 237, intended to illustrate this prin¬ 
ciple. What is the appearance of electrical light through a prism ? What is 
said concerning the different colors of electrical light, when passing between 
surfaces of different kinds 1 










ZZ4 


ELECTRiem. 


955. Several curious little experiments 
are made on this principle. Thus, let 
two cross wires, as in fig. 238, be sus¬ 
pended on a pivot, each having his point 
bent in a contrary direction, and electri¬ 
fied by being placed on the prime con¬ 
ductor of a machine. These points, so 
long as the machine is in action, will 
give off streams of electricity, and as 
the particles of .air repel the points by 
which they are electrified, the little machine will turn round 
rapidly, in the direction contrary to that of the stream of 
electricity. Perhaps, also, the reaction of the atmosphere 
against the current of air given off by the points, assists in 
giving it motion. 

956. When one part or side of an electric is positively, the 
other part or side is negatively electrified. Thus, if a plate 
of glass be positively electrified on one side, it will be nega¬ 
tively electrified on the other, and if the inside of a glass ves¬ 
sel be positive, the outside will be negative. 

957. Advantage of this circumstance is taken, in the con¬ 
struction of electrical jars, called, from the place where they 
were first made, Leyden vials. 

The most common form of this jar is rep¬ 
resented by fig. 239. It consists of a glass 
vessel, coated on both sides up to a, with 
tin foil; the upper part being left naked, so 
as to prevent a spontaneous discharge, or 
the passage of the electric fluid from one 
coating to the other. A metallic rod, rising 
two or three inches above the jar, and ter¬ 
minating at the top with a brass ball, which 
is called the knob of the jar, is made to de¬ 
scend through the cover, till it touches the 
interior coating. It is along this rod that 
the charge of electricity is conveyed to the inner coating, 
while the outer coating is made to communicate with the 
ground. 

958. When a chain is passed from the prime conductor of 
an electrical machine to this rod, the electricity is accumu- 


Describe fig. 233, and explain the principle on which its motion depends ? 
Suppose one part or side of an electric is positive, what will be the electrical 
state of the other side or part ? What part of the electrical apparatus is con 
structed on this principle ? How is the Leyden vial constructed 1 Why is not 
the whole surface of the vial covered with the tin foil ? 


Fig. 239. 



Fig. 238. 














ELECTRICITY. 


325 


kited on the tin foil coating, while the glass above the tin 
foil prevents its escape, and thus the jar becomes charged. 
By connecting together a sufficient number of these jars, any 
quantity .of the electric fluid may be accumulated. For this 
purpose, all the interior coatings of the jars are made to com¬ 
municate with each other, by metallic rods passing between 
them, and finally terminating in a single rod. A similar 
union is also established, by connecting the external coats 
with each other. When thus arranged, the whole series 
may be charged, as if they formed but one jar, and the whole 
series may be discharged at' the same instant. Such a com¬ 
bination of jars is termed an electrical battery. 

959. For the purpose of making a direct communication 
between the inner and outer coating of a single jar, or bat¬ 
tery, by which a discharge is effected, an instrument called 
a discharging rod is employed. It consists of two bent me¬ 
tallic rods, terminated at one end by brass balls, and at the 
other end connected by a joint. This joint is fixed to the 
end of a glass handle, and the rods being movable at the 
joint, the balls can be separated or brought near each other, 
as occasion requires. When opened to a proper distance, 
one ball is made to touch the tin foil on the outside of the 
jar, and then the other is 
brought into contact with the Fi «- 240 - 

knob of the jar, as seen in 
fig. 240. In this manner a 
discharge is effected, or an 
equilibrium produced be¬ 
tween the positive and nega¬ 
tive sides of the jar. 

When it is desired to pass 
the charge through any sub¬ 
stance for experiment, then 
an electrical circuit must be 
established, of which the 

substance to be experimented upon must form a part. That is, 
the substance must be placed between the ends of two metal¬ 
lic conductors, one of which communicates with the positive, 
and the other with the negative side of the jar, or battery. 



How is the Leyden vial charged T In what manner may a number of these 
vials be charged ? What is an electrical battery? Explain the design of fig. 
240, and show how an equilibrium is produced by the discharging rod. When 
)t is desired to pass the electrical fluid through any substance, where must it 
be placed in respect to the two sides of the battery ? 

28 







326 


ELECTRICITY. 


060. When a person takes the electrical shock in the* 
usual manner, he merely takes hold of the chain connected 
with the outside coating, and the battery being charged, 
touches the knob with his finger, or with a metallic rod. 
On making this circuit, the fluid passes through the person 
from the positive to the negative side. 

961. Any number of persons may receive the electrical 
shock, by taking hold of each other’s hands, the first person 
touching the knob, while the last takes hold of a chain con¬ 
nected with the external coating. In this manner, hundreds, 
or perhaps thousands of persons, will feel the shock at the 
same instant, there being no perceptible interval in the dme 
when the first and the last person in the circle feels the sen¬ 
sation excited by the passage of the electric fluid. 

962. The atmosphere always contains more or less elec¬ 
tricity, which is sometimes positive, and at others negative. 
It is, however, most commonly positive, and always so when 
the sky is clear, or free from clouds or fogs. It is always 
stronger in winter than in summer, and during the day than 
during the night. It is also stronger at some hours of the 
day than at others; being strongest about 9 o’clock in the 
morning, and weakest about the middle of the afternoon. 
These different electrical states are ascertained by means of 
long metallic wires extending from one building to another, 
and connected with electrometers. 

963. It was proved by Dr. Franklin, that the electric 
fluid and lightning are the same substance, and this identity 
has been confirmed by subsequent writers on this subject. 

If the properties and phenomena of lightning be compared 
with those of electricity, it will be found that they differ only 
in respect to degree. Thus, lightning passes in irregular 
lines thrc igh the air ; the discharge of an electrical battery 
has the . ime appearance. Lightning strikes the highest 
pointed o' jects—takes in its course the best conductors—sets 
fire to noi conductors, or rends them in pieces—and destroys 
animal li e; all of which phenomena are caused by the 
electric fluid. 

964. Buildings may be secured from the effects of light- 


Suppose the battery is charged, what must a person do to take the shock ? 
What circumstance is related, which shows the surprising velocity with which 
electricity is transmitted ? Is the electricity of the atmosphere positive or 
negative ? At what times does the atmosphere contain most electricity ? How 
are the different electrical states of the atmosphere ascertained ? Who first 
discovered that electricity and lightning are the same ? What phenomena are 
mentioned which belong in common to electricity and lightning ? 



ELECTRICITY. 


327 

rung, by fixing to them a metallic rod, which is elevated 
above any part of the edifice and continued to the moist 
ground, or to the nearest water. Copper, for this purpose, is 
better than iron, not only because it is less liable to rust, but 
because it is a better conductor of the electric fluid. The 
upper part of the rod should end in several fine points, 
which must be covered with some metal not liable to rust, 
such as gold, platina, or silver. No protection is afforded by 
the conductor , unless it is continued without interruption from 
the top to the bottom of the building , and it cannot be relied on 
as a protector , unless it reaches the moist earth, or ends in 
water connected with the earth. Conductors of copper may be 
three-fourths of an inch in diameter, but those of iron should 
be at least an inch in diameter. In large buildings, complete 
protection requires many lightning rods, or that they should 
be elevated to a height above the building in proportion to 
the smallness of their numbers, for modem experiments have 
proved that a rod only protects a circle around it, the radius 
of which is equal to twice its length above the building. 

965. Torpedo. —Some fishes have the power of giving elec¬ 
trical shocks, the effects of which are the same as those 
obtained by the friction of an electric. The best known of 
these are the Torpedo , the Gymnotus electricus and the 
Silurus electricus. 

966. The torpedo, when touched with both hands at the 
same time, the one hand on the under, and the other on the 
upper surface, will give a shock like that of the Leyden 
vial; which shows that the upper and under surfaces of the 
electric organs are in the positive and negative state, like the 
inner and outer surfaces of the electrical jar. 

967. The gymnotus electricus, or electrical eel, possesses 
all the electrical powers of the torpedo, but in a much higher 
degree. When small fish are placed in the water with this 
animal, they are generally stunned, and sometimes killed, 
by his electrical shock, after which he eats them if hungry. 
The strongest shock of the gymnotus will pass a short dis¬ 
tance through the air, or across the surface of an electric, 
from one conductor to another, and then there can be per- 


How may buildings be protected from the effects of lightning ? Which is 
the best conductor, iron or copper ? What circumstances are necessary, that 
the rod may be relied on as a protector ? What animals have the power of 
giving electrical shocks? Is this electricity supposed to differ from that ob¬ 
tained by art ? How must the hands be applied, to take the electrical shock 
of these animals ? 



MAGNETISM 


3*28 

ceired a small but vivid spark of electrical fire; particularly 
if the experiment be made in the dark. x 


MAGNETISM. 

968. The native Magnet, or Loadstone, is an ore of iron, 
which is found in various parts of the world. Its color is 
iron black ; its specific gravity from 4 to 5, and it is some¬ 
times found in crystals. This substance, without any prepa¬ 
ration, attracts iron and steel, and when suspended by a 
string, will turn one of its sides towards the north and 
another towards the south. 

969. It appears that an examination of the properties of 
this species of iron ore, led to the important discovery of the 
magnetic needle, and subsequently laid the foundation for the 
science of magnetism ; though at the present day magnets 
are made without this article. 

970. The whole science of magnetism is founded on the 
fact, that pieces of iron or steel, after being treated in a certain 4 
manner, and then suspended, will constantly turn one of their 
ends towards the north, and consequently the other towards 
the south. The same property has been more recently 
proved to belong to the metals nickel and cobalt , though 
with much less intensity. 

971. The poles of a magnet are those parts which possess 
the greatest power, or in which the magnetic virtue seems 
to be concentrated. One of the poles points north, and the 
other south. The magnetic meridian is a vertical circle in 
the heavens, which intersects®:the horizon at the points to 
which the magnetic needle, when at rest, directs itself. 

972. The axis of a magnet, is a right line which passes 
from one of its poles to the other. 

The equator of a magnet, is a line perpendicular to its axis, 
and is at the centre between the two poles. 

973. The leading properties of the magnet are th? follo\v- 
ing. It attracts iron and steel, and when suspended so as to 


What is the native magnet or loadstone ? What are the properties of the * 
loadstone? On what is the whole subject of magnetism founded? What 
other metals besides iron possess the magnetic property ? What are the poles 
of a magnet ? What is the axis of a magnet ? What is the equator of a mag 
net? 






MAGNETISM. 


829 


move freely, it arranges itself so as to point north and south . 
this is called the polarity of the magnet. When the south 
pole of one magnet is presented to the north pole of another, 
they will attract each other; this is called magnetic attrac¬ 
tion. But if the two north or two south poles be brought to¬ 
gether, they will repel each other, and this is called magnetic, 
repulsion. When a magnet is left to move freely, it does not 
lie in a horizontal direction, but one pole inclines downwards, 
and consequently the other is elevated above the line of the 
horizon. This is called the dipping , or inclination of the 
magnetic needle. Any magnet is capable of communicating 
its own properties to iron or steel, and this, again, will impart 
its magnetic virtue to another piece of steel, and so on indefi¬ 
nitely. 

974. If a piece of iron or steel be brought near one of the 
poles of a magnet, they will attract each other, and if suffer¬ 
ed to come into contact, will adhere so as to require force to 
separate them. This attraction is mutual; for the iron at¬ 
tracts the magnet with the same force that the magnet at¬ 
tracts the iron. This may be proved, by placing the iron 
and magnet on pieces of wood floating on water, when they 
will be seen to approach each other mutually. 

975. The force of magnetic attraction varies with the dis¬ 
tance in the same ratio as the force of gravity ; the attracting 
force being inversely as the square of the distance between 
the magnet and the iron. 

976. The magnetic force is not sensibly affected by the 
interposition of any substance except those containing iron, 
or steel. Thus, if two magnets, or a magnet and piece of 
iron, attract each other with a certain force, this force will be 
the same, if a plate of glass, wood, or paper, be placed be¬ 
tween them. Neither will the force be altered, by placing 
the two attracting bodies under water, or in the exhausted 
receiver of an air pump. This proves that the magnetic in¬ 
fluence passes equally well through air, glass, wood, paper, 
water, and a vacuum. 

977. Heat weakens the attractive power of the magnet, 
and a white heat entirely destroys it. Electricity will change 
the poles of the magnetic needle, and the explosion of a 

What is meant by the polarity of a magnet ? When do two magnets attract 
and when repel each other ? What is understood by the dipping of the mag¬ 
netic needle ? How is it proved that the iron attracts the magnet with the same 
force that the magnet attracts the iron ? How does the force of magnetic at¬ 
traction vary with the distance ? Does the magnetic force vary with the in¬ 
terposition of any substance between the attracting bodies? W ;» live <- 
feet of heat on the magnet ? 

28* 




MAGNETISM. 


33t» 

small quantity of gun-powder on one of the poles, will have 
the same effect. 

978. The attractive power of the magnet may be increased 
by permitting a piece of steel to adhere to it, and then sus¬ 
pending to the steel a little additional weight every day, for 
it will sustain, to a certain limit, a little more weight on one 
day than it would on the day before. 

979. Small natural magnets will sustain more than large 
ones in proportion to their weight. It is rare to find a natural 
magnet, weighing 20 or 30 grains, which will lift more than 
thirty or forty times its own weight. But a minute piece of 
natural magnet, worn by Sir Isaac Newton, in a ring, which 
weighed only three grains, is said to have been capable of 
lifting 746 grains, or nearly 250 times its own weight. 

980. The magnetic property may be communicated from 
the loadstone, or artificial magnet, in the following manner, 
it being understood that the north pole of one of the magnets 
employed, must always be drawn towards the south pole of 
the new magnet, and that the south pole of the other mag¬ 
net employed, is to be drawn in the contrary direction. The 
north poles of magnetic bars are usually marked with a line 
across them, so as to distinguish this end from the other. 

981. Place two mag¬ 
netic bars, a and 6, fig. 

241, so that the north 
end of one may be near¬ 
est the south end of the 
other, and at such a dis¬ 
tance that the ends of 
the steel bar to be touch¬ 
ed, may rest upon them. 

Having thus arranged them, as shown in the figure, take 
the two magnetic bars, d and e, and apply the south end of 
e , and the north end of rf, to the middle of the bar eleva¬ 
ting their ends as seen in the figure. Next separate the bars 
e and d, by drawing them in opposite directions along the 

. surface of c, still preserving the elevation of their ends ; then 
removing the bars d and e to the distance of a foot or more 
from the bar c, bring their north and south poles into contact, 
and then having again placed them on the middle of c, draw 
them in contrary directions, as before. The same process 

What is the effect of electricity, or the explosion of gun-powder on it? Hour 
may the power of a magnet be increased ? What is said concerning the com¬ 
parative powers of great and small magnets? Explain fig. 241, and describe 
the mode of making a magnet 


Fig. 241. 








MAGNETISM. 


331 


must be repeated many times on each side of the bar c, when 
it will be found to have acquired a strong and permanent 
magnetism. 

982. If a bar of iron be placed, for a long period of time, 
in a north and south direction, or in a perpendicular position, 
it will often acquire a strong magnetic power. Old tongs, 
pokers, and fire shovels, almost always possess more or less 
magnetic virtue, and the same is found to be the case with 
the iron window bars of ancient houses, whenever they have 
happened to be placed in the direction of the magnetic line. 

983. A magnetic needle , such as is employed in the mari¬ 
ner’s and surveyor’s compass, may be made by fixing a piece 
of steel on a board, and then drawing two magnets from the 
centre towards each end, as directed at fig. 241. Some mag¬ 
netic needles in time lose their virtue, and require again to 
be magnetized. This may be done by placing the needle, 
still suspended on its pivot, between the opposite poles of two 
magnetic bars. While it is receiving the magnetism, it 
will be agitated, moving backwards and forwards, as though 
it were animated, but when it has become perfectly mag¬ 
netized, it will remain quiescent. 

984. The dip, or inclination of the magnetic needle, is its 
deviation from its horizontal position, as already mentioned. 
A piece of steel, or a needle, which will rest on its centre, 
in a direction parallel to the horizon, before it is magnetized, 
will afterwards incline one of its ends towards the earth. 
This property of the magnetic needle was discovered by a 
compass maker, who, having finished his needles before they 
were magnetized, found that immediately afterwards, their 
north ends inclined towards the earth, so that he was obliged 
to add small weights to their south poles, in order to make 
them balance, as before. 

985. The dip of the magnetic needle is measured, by a 
graduated circle, placed in the vertical position, with the 
needle suspended by its side. Its inclination from a hori¬ 
zontal line, marked across the face of this circle, is the mea¬ 
sure of its dip. The circle, as usual, is divided into 360 
degrees, and these into minutes and seconds. 

986. The dip of the needle does not vary materially at the 
same place, but differs in different latitudes, increasing as it 


In what positions do bars of iron become magnetic spontaneously? How 
may a needle be magnetized without removing it from its pivot ? How was 
the dip of the magnetic needle first discovered ? In what manner is the dip 
neasured ? What circumstance increases or diminishes the dip of the needle ? 



MAUNETLSM 


332 

is carried towards the north, and diminishing as it is earned 
towards the south. At London, the dip for many years has 
varied little from 72 degrees. In the latitude of 80 degrees 
north, the dip, according to the observations of Capt. Parry, 
was 88 degrees. 

987. Although, in general terms, the magnetic needle is 
said to point north and south, yet this is very seldom strictly 
true, there being a variation in its direction, which differs in 
degree at different times and places. This is called the varia¬ 
tion , or declination , of the magnetic needle. 

988. This variation is determined at sea, by observing the 
different points of the compass at which the sun rises, or 
sets, and comparing them with the true points of the sun’s 
rising or setting, according to astronomical tables. By such 
observations it has been ascertained that the magnetic needle 
is continually declining alternately to the east or west from 
due north, and that this variation differs in different parts of 
the world at the same time, and at the same place at differ¬ 
ent times. 

989. In 1580, the needle at London pointed 11 degrees 15 
minutes east of north, and in 1657 it pointed due north and 
south, so that it moved during that time at the mean rate of 
about 9 minutes of a degree in each year, towards the north. 
Since 1657, according to observations made in England, it 
has declined gradually towards the west, so that in 1803, its 
variation west of north was 24 degrees. 

990. At Hartford, Connecticut, in latitude about 41, it 
appears from a record of its variations, that since the year 
1824, the magnetic needle has been declining towards the 
west, at the mean rate of 3 minutes of a degree annually, and 
that on the 20th of July, 1829, the variation was 6 degrees 
3 minutes west of the true meridian. 

991. The cause of this annual variation has not been de¬ 
monstrated, though according to the experiment of Mr. Can¬ 
ton, it has been ascertained that there are slight variations 
during the different months of the year, which seem to de¬ 
pend on the degrees of heat and cold. 

992. The directive power of the magnet is of vast impor¬ 
tance to the world, since by this power, mariners are enabled 
to conduct their vessels through the widest oceans, in any 


What is meant by the declination of the magnetic needle ? How is this 
variation determined? What has been ascertained concerning the variation 
of the needle at different times and places? 



GALVANISM. 


333 


given direction, and by it travellers can find their way across 
deserts which would otherwise be impassable. 


GALVANISM. 

993. The design of this epitome of the principles of Gal¬ 
vanism, is to prepare the pupil to understand the subject of 
Electro-Magnetism, which, on account of several recent pro¬ 
positions to apply this power to the movement of machinery, 
has become one of the exciting scientific subjects of the day. 

We shall therefore leave the student to learn the history 
and progress of Galvanism from other treatises, and come at 
once to the principles of the science. 

994. When two metals, one of which is more easily oxi¬ 
dated than the other, are placed in acidulated water, and the 
two metals are made to touch each other, or a metallic com¬ 
munication is made between them, there is excited an elec¬ 
trical or galvanic current, which passes from the metal most 
easily oxidated, through the water, to the other metal, and 
from the other metal through the water around to the first 
metal again, and so in a perpetual circuit. 

995. If we take, for example, a 
slip of zinc, and another of copper, 
and place them in a cup of diluted 
sulphuric acid, fig. 242, their up¬ 
per ends in contact, and above the 
water, and their lower ends sepa¬ 
rated, then there will be constituted 
a galvanic circle , of the simplest 
form, consisting of three elements, 
zinc, acid, copper. The galvanic 
influence being excited by the acid, 
will pass from the zinc, Z, the metal most easily oxdiated, 
through the acid, to the copper C, and from the copper to 
the zinc again, and so on continually, until one or the other 
of the elements is destroyed, or ceases to act. 

996. The same effect will be produced, if instead of allow¬ 
ing the metallic plates to come in contact, a communication 
between them be made by means of wires, as shown by fig. 


What conditions are necessary to exeite the galvanic action? From which 
metai does the galvanism proceed? Describe the circuit. 


Fig. 242. 










334 


GALVANISM. 


243 In this case, as well Fi s- 243 * 

as‘in the former, the elec¬ 
tricity proceeds from the 
zinc Z, which is the posi¬ 
tive side, to the copper C, 
being conducted by the 
wires in the direction 
shown by the arrows. 

997. The completion of 
the circuit by means of 
wires, enables us to make 
experiments on different 
substances by passing the 
galvanic influence through 
them, this being the method employed to exhibit the effects 
of galvanic batteries, and by which the most intense heat 
may be produced. 



ELECTRO-MAGNETISM. 

998. When the two poles of a battery are connected by 
means of a copper wire of a yard or two in length, the 
two parts being supported on a table in a north and south 
direction, for some of the experiments, but in others the di¬ 
rection must be changed as will be seen. This wire, it will 
be remembered, is called the uniting wire. 

999. Being thus prepared, and the galvanic battery in 
action, take a magnetic needle six or eight inches long, pro¬ 
perly balanced on its pivot, and having detached the wire 
from one of the poles, place the magnetic needle under the 
wire, but parallel with it, and having waited a moment for 
the vibratidns to cease, attach the uniting wire to the pole. 
The instant this is done, and the galvanic circuit completed, 
the needle will deviate from its north and south position, 
turning towards the east or west, according to the direction 
in which the galvanic current flows. If the current flows 
from the north, or the end of the wire along which it passes 
to the south is connected with the positive side of the battery, 
then the north pole of the needle will turn towards the east; 


What is the uniting wire ? If the needle is stationary, and the current 
flows from the north, what way will the needle turn ? 



















ELECTROMAGNETISM. 335 

hut if the direction of the current is changed, the same pole 
will turn in the opposite direction. 

1000. If the uniting wire is placed under the needle, in¬ 
stead of over it, as in the above experiment, the contrary 
effect will be produced, and the north pole will deviate to¬ 
wards the west. 

1001. These deviations 
will be understood by the fol¬ 
lowing figures. In fig. 244, 

N presents the north, and S 
the south pole of the mag¬ 
netic needle, and p the posi¬ 
tive and n the negative ends 
of the uniting wire. The 
galvanic current, therefore, 
flows from p towards n, or, 
the wire being parallel with 
the needle, from the north towards the south, as shown by 
the direction of the arrow in the figure. 

Now the uniting wire beiug above the needle, the pole N, 
which is towards the positive side of the battery, will devi¬ 
ate towards the east, and the needle will assume the direction 
N' S'. 

On the contrary, when the uniting wire is carried below the 
needle, the galvanic current being in the same direction as 
before, as shown by fig. 245, then the same, or north pole, 
will deviate towards the west, or in the contrary direction 
from the former, and the needle will assume the position 
N S. 

1002. When the uniting 
wire is situated in the same 
horizontal plane with the 
needle, and is parallel to it, 
no movement takes place to¬ 
wards the east or west; but 
the needle dips, or the end 
towards the positive end of 
the wire is depressed, when 
the wire is on the east side, 
and elevated-when it is on the 
west side. 


t 

Fig. 245. 




Explain fig. 244. 







336 


ELECTROMAGNETISM. 



Thus, if the uniting wire p 
7 i, fig. 246, is placed on the 
east side of the needle N S, 
and parallel to, and on a level 
with it, then the north pole, 

N, being towards the positive 
end of the wire, will be ele¬ 
vated, and the needle will as¬ 
sume the position of the dotted 
needle N' S'. But if the wire 
be changed to the western 

side, other circumstances being the same, then the north pole 
will be depressed, and the needle will take the direction of 
the dotted line N" S". 

1003. If the uniting wire, instead of being parallel to the 

needle, be placed at right-angles with it, that is, in the direc¬ 
tion of east and west, and the needle brought near, whether 
above or below the wire, then the pole is depressed when the 
positive current is from the west, and elevated when it is 
from the east. & 

1004. Thus, the pole S, ■ Fig- 247. 

fig. 247, is elevated, the cur¬ 
rent of positive electricity 
being from p to n, that is, 
across the needle from the 
east towards the west. If 
the direction of the positive 
current is changed, and 
made to flow from n to p, 
the other circumstances 
being the same, the south 
pole of the needle will be depressed 

1005. When the uni¬ 
ting wire, instead of be¬ 
ing placed in a horizon¬ 
tal position as in the last 
experiment, is placed ver¬ 
tically, either to the north 
or south of the needle 
and near its pole, 
shown by fig. 248, then 
if the lower extremity of 



Fig. 248. 


as 


V 


TV 





Explain figures 246, 247, and 248. 











ELECTRO-MAGNETISM. 


S37 


the wire receives the positive current, as from p ton, the needle 
will turn its pole towards the west. 

If now the wire be made to cross the needle at a point 
about half way between the pole and the middle, the same 
pole will deviate towards the east. If the positive current be 
made to flow from the upper end of the wire, all these phe¬ 
nomena will be reversed. 

LAWS OF ELECTRO-MAGNETIC ACTION. 

1006. An examination of the facts which may be drawn 
from an attentive consideration of the above experiments are 
sufficient to show that the magnetic force which emanates 
from the conducting wire, is different in its operation from 
any other force in nature, with which philosophers had been 
acquainted. 

1007. This force does not act in a direction parallel to that 
of the current which passes along the wire, u but its action 
produces motion in a circular direction around the wire, that 
is, in a direction at right-angles to the radius, or in the direc¬ 
tion of the tangent to a circle described round the wire in a 
plane perpendicular to it.” 

1008. In consequence of this circular current, which seems 
to emanate from the regular polar currents of the battery, 
the magnetic needle is made to assume the positions indicated 
by the figures above described, and the effects of which is, 
to change the direction of the needle from the magnetic 
meridian, moving it through the section of a circle in a di¬ 
rection depending on the relative position of the wire and the 
course of the electric fluid. And we shall see hereafter that 
there is a variety of methods by which this force can be ap¬ 
plied to produce a continued circular motion. 

CIRCULAR MOTION OF THE ELECTRO-MAG¬ 
NETIC FLUID. 

1009. We have already stated that the action of this fluid 
produces motion in a circular direction. Thus, if we sup¬ 
pose the conducting wire to be placed in a vertical situation, 
as shown by fig. 249, and p n, the current of positive electri¬ 
city, to be descending through it, from p to n, and if through 
the point c in the wire the plane N N be taken, perpendicular 
to p », that is in the present case a horizontal plane, then if 


Does the magnetic force of galvanism differ from any lo.ce before known, 
or not T In what direction does this force act, as it passes along the wire • 


29 





33P 


ELECTRO-MAGNETISM. 


any number of circles 
be described in that 
plane, having c for their 
common centre, the ac¬ 
tion of the current on 
the wire upon the north 
poie of the magnet, 
will be to move it in 
a direction correspond¬ 
ing to the motion of 
the hands of a watch, 
having the dial to¬ 
wards the positive pole 
of the battery. The 
arrows show the di¬ 
rection of the current’s 
motion in the figure. 

If we employ a metal through the substance of which the mag 
netic needle can move, we shall have an opportunity of know¬ 
ing whether the fluid has the circular action in question, for 
then the needle will have liberty to move in the direction of 
the electrical current. 

1010. For this purpose mercury is well adapted, being a 
good conductor of electricity, and at the same time so fluid 
as to allow a solid to circulate in it, or on its surface, with 
considerable facility. This, therefore, is the substance em¬ 
ployed in these experiments. 


Fig. 249. 



VIBRATION OF A WIRE. 


1011. A conducting copper 
wire, Wj fig. 250, is suspended 
by a loop from a hook of the 
same metal, which passes 
through the arm of metal or 
wood, as seen in the cut. The 
upper end of the hook termi¬ 
nates in the cup P, to contain 
mercury. The lower end of 
the copper wire just touches 
the mercury, Q,, contained in 
a little trough about an inch 
long, formed in the wood on 
which the horse-shoe magnet, 
M, is laid, the mercury being 
equally distant from the two 
poles. 


Fig. 250. 



0 



















ELECTRO-MAGNETISM. 


339 

The cup, N, has a stem of wire which passes througn 
the wood of the platform into the mercury, this end of the 
wire being tinned, or amalgamated, so as to form a perfect 
contact. 

1012. Having thus prepared the apparatus, put a little 
mercury into the cups P and N, and then form the galvanic 
circuit by placing the poles of the battery in the two cups, 
and if every thing is as it should be, the wire will begin to 
vibrate, being thrown with considerable force either towards 
M or Q,, according to the position of the magnetic poles, or 
the direction of the current, as already explained. In either 
case it is thrown out of the mercury, and the galvanic cir¬ 
cuit being thus broken, the effect ceases until the wire falls 
back again by its own weight, and touches the mercury, 
when the current being again perfected, the same influence 
is repeated, and the wire is again thrown away from the 
mercury, and thus the vibratory motion becomes constant. 

This forms an easy and beautiful electro-magnetic experi¬ 
ment, and may be made by any one of common ingenuity, 
who possesses a galvanic battery, even of small power, and 
a good horse-shoe magnet. 

1013. The platform may be nothing more than a piece of 
pine board eight inches long and six wide, with two sticks 
of the same wood, forming a standard and arm for suspend¬ 
ing the vibrating wire. The cups may be made of percussion 
caps, exploded, and soldered to the ends of pieces of copper 
bell wire. 

1014. The wire must be nicely adjusted with respect to 
the mercury, for if it strikes too deep, or is too far from the 
surface, no vibrations will take place. It ought to come so 
near the mercury as to produce a spark of electrical fire, as 
it passes the surface, at every vibration, in which case it may 
be known that the whole apparatus is well arranged. The 
vibrating wire must be pointed and amalgamated, and may 
be of any length, from a few inches to a foot or two. 

ROTATION OF A WHEEL. 

1015. The same force which throws the wire away from 
the mercury, will cause the rotation of a spur-wheel. For 
this purpose the conducting wire, instead of being suspend¬ 
ed as in the former experiment, must be fixed firmly to the 

How may the direction of the vibrating wire be changed? Explain fig. 250, 
and describe the course of the electric fluid from one cup to the other.. How 
must the points of the vibrating wire be adjusted in order to act. 



ELECTRO MAGNETISM. 


340 

arm, as shown by fig. 251. 

A support for the axis of the 
wheel may he made by sol¬ 
dering a short piece to the side 
of the conducting wire, so as 
to make the form of a fork, 
the lower ends of which must 
he flattened with a hammer, 
and pierced with fine orifices, 
to receive the ends of the 
axis. 

1016. The apparatus for a 
revolving wheel is in every re¬ 
spect like that already descri¬ 
bed for the vibrating wire, ex¬ 
cept in that above noticed. 

The wheel may be made of 
brass or copper, but must be 
thin and light, and so suspend¬ 
ed as to move freely and easily. The points of the notches 
must be amalgamated, which is done in a few minutes, by 
placing the wheel on a flat surface, and rubbing them with 
mercury by means of a cork. A little diluted acid from the 
galvanic battery will facilitate the process. The wheel may 
be from half an inch to several inches in diameter. A cent 
hammered thin, which may be done by heating it two or 
three times during the process, and then made perfectly 
round, and its diameter cut into notches with a file, will 
answer every purpose. 

1017. This affords a striking and novel experiment; for 
when every thing is properly adjusted, the wheel instantly 
begins to revolve by touching with one of the wires of the 
battery the mercury in the cup P or N. 

When the poles of the magnet, or those of the battery, 
are changed, the wheel instantly revolves in a contrary di 
rection from what it did before. 

1018. It is, however, not absolutely necessary to divide 
the wheel into notches, or rays, in order to make it revolve, 
though the motion is more rapid, and the experiment sue 
ceeds much better by doing so. 

Explain fig. 251 ? In what manner may the points of the spur wheel be 
amalgamated ? If the motion of the fluid is changed, what effect does it have 
•n the wheel ? 


Fig. 251. 










ELECTROMAGNETISM. 


341 


REVOLUTION OF TWO WHEELS. 

1016. If two wheels 
be arranged as repre¬ 
sented by fig. 252, 
they will both revolve 
by the same electrical 
current. Each horse- 
shoe magnet has its trough of mercury. The magnets have 
been omitted in the drawing, but are to be placed precisely 
as in the last figure. The electrical communication is to be 
made through the cups of mercury, P and N, and its course 
is as follows :—From the cup it passes into the mercury • 
from the mercury through the radii to the axis of the wheel’ 
and along the axis to the other wheel, down which it passes 
to the mercury, and so to the other cup, and to the opposite 
pole of the battery. 

The poles of the magnets for this experiment, must be op¬ 
posed to each other. 

ELECTRO-MAGNETIC INDUCTION. 

1020. Experiment proves that the passage of the galvanic 
current through a copper wire renders iron magnetic when 
in the vicinity of the current. This is called magnetic in¬ 
duction. 

1021. The appara¬ 
tus for this purpose is 
represented by fig. 

253, and consists of a 
copper wire coiled, by 
winding it around a 
piece of wood. The 
turns of the wire 
should be close together for actual experiment, they being 
parted in the figure to show the place of the iron to be mag¬ 
netized. The best method is, to place the coiled wire, which 
is called an electrical helix , in a glass tube, the two ends of 
the wire of course projecting. Then placing the body to be 
magnetized within the folds, send the galvanic influence 
through the whole by placing the poles of the battery in the 
cups. 

Explain fig. 252, and show how two wheels may he made to rerolve by the 
same current. What is meant by magnetic induction? Explain fig. 253. 
Wl.at is this figure called ? Does any substance become permanently mag 
netic by the action of the electrical helix ? 

29* 


Fig. 253. 









342 


ELECTROMAGNETISM. 


1022. Steel thus becomes permanently magnetic, the 
poles, however, changing as often as the fluid is sent through 
it in a contrary direction. A piece of watch-spring placed in 
the helix, and then suspended, will exhibit polarity, but if 
its position be reversed in the helix, and the current again 
sent through it, the north pole will become south. If one 
blade of a knife be put into one end of the helix, it will re¬ 
pel the north pole of a magnetic needle, and attract the 
south ; and if the other blade be placed in the opposite end 
of the helix, it will attract the north pole, and repel the south, 
of the needle. 

1023. Temporary Magnets .—Temporary magnets, of al¬ 
most any power, may be made by winding a thick piece of 
soft iron with many coils of insulated copper wire. 

The best form of a magnet for this purpose is that of a 
horse-shoe, and which may be made in a few minutes by 
heating and bending a piece of cylinder iron, an inch or two 
in diameter, into this form. 

1024. The copper wire (bell wire) may be insulated by 
winding it with cotton thread. If this cannot be procured, 
common bonnet wire will do, though it makes less powerful 
magnets than copper. 

1025. The coils of 
wire may begin near 
one pole of the magnet 
and terminate near the 
other, as represented 
by fig. 254, or the wire 
may consist of shorter 
pieces wound over each 
other, on any part of 
the magnet. In either 
case, the ends of the 
wire, where several 
pieces are used, must 
be soldered to two 
strips of tinned sheet 
copper, for the com¬ 
bined positive and ne¬ 
gative poles of the 
wires. To form the magnet, these pieces of copper are made 


How may the poles of a magnet be changed by the helix ? How may tem¬ 
porary magnets be made ? For what purpose are the ends of the wires to be 
soldered to pieces of copper? 


Fig. 254. 







ELECTROMAGNETISM. 


343 


to communicate with the poles of the battery, by means of 
cups containing mercury, as shown in the figure, or by any 
other method. 

1026. The effect is surprising, for on completing the cir¬ 
cuit with a piece of iron an inch in diameter, in the proper 
form, and properly wound, a man will find it difficult to pull 
off the armature from the poles ; but on displacing one of the 
galvanic poles, the attraction ceases instantly, and the man, 
if not careful, will fall backwards, taking the armature with 
him. Magnets have been constructed in this manner, which 
would suspend ten thousand pounds. 

1027. Galvanic Battery. —One of the most convenient 
forms of a galvanic battery for experiments described in this 
work is represented by fig. 255. 

It consists of a cylinder of Fig. 255 . 

sheet copper, within which is 
another of zinc. The zinc 
has for its bottom a piece of 
sheep skin, or bladder, tied on 
with a string, and is suspend¬ 
ed an inch or two from the 
bottom of the copper cylinder. 

Or, the whole inner cylinder 
may be made of leather with 
a slip of zinc within it. This 
is done to prevent the fluid 
which the inner cylinder con¬ 
tains from mixing with that 
contained between the two; 
and still, the leather being 
porous, the water it contains 
conducts the galvanic influ¬ 
ence from one cell to the oth¬ 
er, as already stated. The diameter of the outer cup may be 
five or six inches, and the inner one three or four. The zinc 
may be suspended by making two holes near the top and 
tying on a piece of glass tube or a slip of wood. This part 
has often to be removed and cleaned, by scraping off the 
black oxide, which, if it remains, will prevent the action of 
the battery. The action will be sustained much longer if 
the zinc is amalgamated by spreading on it a little mercury 
before it is used, and while the surface is bright. 

The cups, P N, are the positive and negative poles. They 



describe the battery fig. 255. Which is the positive, and which the nega¬ 
tive metal ? 















344 


ELECTRO-MAGNETISM. 


may be made of percussion caps, soldered to the ends of two 
copper wires ; the other ends being connected by soldering, 
or otherwise, one with the zinc, and the other with the cop¬ 
per, cylinder. 

The inner cup is to be filled with water mixed with about 
a twentieth part of sulphuric acid, while the cell between the 
two contains a saturated solution of sulphate of copper, or blue 
vitriol. In order to keep the solution saturated, especially 
when casts are to be taken, some of the solid vitriol is to be 
tied in a rag and suspended in it. 

This battery, it will be seen, differs materially from that 
already described. In that the galvanic fluid is only availa¬ 
ble for the purpose there described, while from this the influ¬ 
ence may be applied to any purpose required. 

ELECTROTYPE. 

The art of covering the base metals, as copper, and the 
alloys of zinc, tin, &c., with gold and silver, as also of copy¬ 
ing medals, by means of the electrical current, is called elec¬ 
trotype or voltatype. 

This new art is founded on the simple fact, that when the 
galvanic influence is passed through a metallic solution, 
under certain conditions, decomposition takes place, and the 
metal is deposited in its pure form on the negative pole of the 
battery. 

The theory by which this effect is explained is, that the 
hydrogen evolved by the action of the acid on the positive 
pole of the battery combines with the oxygen of the dissolved 
metal, forming water, while the metal itself, thus set free, is 
deposited at the negative side of the battery. 

Many of the base metals, as copper, the alloys of zinc, and 
tin, may by such means be covered with gold, or silver, and 
thus a cheap and easy method of gilding and plating is 
effected. 

This art, now only about four years old, has excited great 
interest, not only among men of science, but among me¬ 
chanics, so that in England many hundreds, and perhaps 
thousands of hands are already employed in silvering, gild¬ 
ing, and coppering, taking impressions of medals and of cop¬ 
per plates, for printing, and of performing such other work as 
the art is capable of. Yolumes have been written to explain 


What is electrotype ? On what fact is it said this art is foundi ) ? On 
which pole is the metal deposited ? What is the theory by which this effect is 
explained? 



ELECTRO-MAGNETISM. 


345 


the different processes to which this art is applicable, and 
considering its recent discovery and the variety of uses to 
which it is already applied, no doubt can exist that it will 
finally become of great importance to the world. 

In this short treatise we can only introduce the pupil to 
the subject, by describing a few of the most simple processes 
of the art in question, and this we hope to do in so plain a 
manner, that any one of common ingenuity can gild, silver, 
or copper, and take impressions of medals at his leisure. 

Copying of Medals .—This new art has been applied very 
extensively in the copying of ancient coins and medals, 
which it does in the utmost perfection, giving every letter, 
and feature, or even an accidental scratch, exactly like the 
original. When the coin is a cameo the figures or letters 
being raised, it is obvious that if the metal be cast directly 
upon it, the medal will be reversed, that is, the figures will be 
indented, and the copy will be an intaglio instead of a cameo. 
To remedy this, a cast, or impression must first be taken of 
the medal, on which the electrotype process is to act, when 
the copy will in all respects imitate the original. 

There is a variety of ways of making such casts, accord¬ 
ing to the substance used for the purpose. We shall only 
mention plaster of Paris, wax, and fusible metal. 

Plaster Casts .—When plaster is used, it must be, what 
is termed boiled, that is, heated, so as to deprive it of all 
moisture. This is the preparation of which stereotype casts 
are made. The dry powder being mixed with water to the 
consistence of cream, is placed on the medal with a knife to 
the thickness of a quarter or half an inch, according to its 
size. In a few minutes the plaster sets , as it is termed, or 
becomes hard. To insure its easy detachment, the medal is 
rubbed over with a little oil. 

The cast thus formed is first to be coated with boiled 
linseed oil, and then its face covered with fine pulverized 
black lead, taking care that the indented parts are not filled, 
nor the raised parts left naked. The lead answers the pur¬ 
pose of a metallic surface, on which the copper is deposited by 
the galvanic current. This is a curious, and very convenient 
discovery, since wood cuts, engraved stones, and copies in 
sealing wax, can thus be copied. 

To insure contact between the black lead on the face of 
che cast and the wire conductor, the cast is to be pierced 
with an awl, on one of its edges, and the sharp point of the 
wire passed to the face, taking care, after this is done, to rub 


340 


ELECTROMAGNETISM. 


on more lead, so that it shall touch the point of the wire, and 
thus communicate with the whole face of the medal. 

Wax Casts .—To copy medallions of plaster of Paris, 
place the cast in warm water, so that the whole may be sat¬ 
urated with the water, but keeping the face above it. W hen 
the cast has become warm and moist, remove, and having 
put a slip of paper around its rim, immediately pour into the 
cup thus formed, bees wax, ready melted for this purpose. 
In this way copies may be taken, not only from plaster casts, 
but from those of other substances. 

To render the surface of the wax a conductor of electricity, 
it is to be covered with^lack lead in the manner directed for 
plaster casts. This is put on with a soft brush, until it be¬ 
comes black and shining. 

The electrical conductor is now to be heated and pressed 
upon the edge of the wax, taking care that a little of its sur¬ 
face is left naked, on, and around which the black lead is 
again to be rubbed, to insure contact with the whole surface. 

Both of the above preparations require considerable inge¬ 
nuity and attention in order to make them succeed in receiv¬ 
ing the copper. If the black lead does not communicate 
with the pole, and does not entirely cover the surface, or if it 
happens to be a poor quality, which is common, the process 
will not succeed ; but patience, and repeated trials, with at¬ 
tention to the above descriptions, will insure final success. 

Fusible Metal Casts .—This alloy is composed of 8 parts of 
bismuth, 5 of lead, and 3 of tin, melted together. It melts 
at about the heat of boiling water, and hence may be used in 
taking casts from engraved stones, coins, or such other sub¬ 
stances as a small degree of heat will not injure. 

To take a cast with this alloy, surround the edge of the 
medal to be copied, with a slip of paper, by means of paste, 
so as to form a shallow cup, the medal being the bottom. 
Then having melted the alloy in a spoon, over an alcohol 
lamp, pour it in, giving it a sudden blow on the table, or a 
shake, in order to detach any air, which may adhere to the 
medal. In a minute or two it will be cool, and ready for the 
process. 

Another method is, to attach the medal to a stick, with 
sealing wax, and having poured a proper quantity of the 
fused alloy on a smooth board, and drawn the edge of a card 
over it, to take off the dross, place the medal on it, and with 
a steady hand let it remain until the cast cools. 

Next, having the end of the coppei wire for the zinc 


ELECTROMAGNETISM. 


347 


}»ole clean, lieat it over a lamp, and touch the edge of tho 
cast therewith, so that they shall adhere, and the cast will 
now be ready for the galvanic current. 

To those who have had no experience in the electrotype 
art, this is much the best, and most easy method of taking 
copies, as it is not liable to failure like those requiring the 
surfaces of the moulds to be black leaded, as above described. 

Galvanic Arrangement .—Having prepared the moulds, as 
above directed, these are next to be placed in a solution of 
the sulphate of copper, (blue vitriol) and subjected to the 
electrical current. For this purpose only a very simple bat¬ 
tery is required, especially where tjie object is merely a mat¬ 
ter of curiosity. 

For small experiments, a glass jar ho’ding a pint, or a 
pitcher, or even a tumbler will answer, \o hold the solution. 
Provide also a cylinder of glass two inches in diameter and 
stop the bottom with some moist planter of Paris, or instead 
thereof, tie around it a piece of bladder, or thin leather, or 
the whole cylinder may be made of leather, with the edges 
sewed nicely together, and stopped with a cork, so that it 
will 4 iot leak. The object of this part of the arrangement is, 
to keep the dilute sulphuric acid which this contains, from 
mixing with the solution of sulphate of copper, which sur¬ 
rounds it, still having the texture of this vessel so spongy as 
to allow the galvanic current to pass through the moisture 
which it absorbs, water being a good conductor of electricity. 

Provide also a piece of zinc in form of a bar, or cylinder, 
or slip, of such size as to pass freely into the above described 
cylinder. 

Having now the materials, the arrangement Fi s- 256 - 
will readily be understood by fig. 256, where 
c is the vessel containing the solution of sul¬ 
phate of copper; a, the cylinder of leather, or 
glass ; ^ the zinc, to which a piece of copper 
wire is fastened, and at the other end of which, 
is the cast, m, to be copied. The proportions 
for the vessel, a, are about 1 part sulphuric 
acid to 16 of water by measure. The solu¬ 
tion of copper for c may be in the proportions 
of 2 ounces of the salt to 4 ounces of water. 

The voltaic current passes from the positive 
zinc to the negative amalgam cast, where the pure copper is 
deposited. 

In order to keep the solution saturated, a little sulphate of 



n 



... 

1 z 

— 



lr 
























348 


ELECTRO-MAGNETISM. 


copper is tied in a rag, and suspended in the solution. In 
24 or 36 hours, the copper, (if all is right,) will be sufficiently 
thick on the cast, the back and edges of which should be 
covered with varnish to prevent its deposition except on the 
face. 

If the copper covers the edges, a file or knife will remove 
it, when by inserting the edge of the knife between the two 
metals, the copy will be separated, and will be found an ex¬ 
act copy of the original. 

If the acid in the inner cylinder is too strong, the process 
is often too vigorous, and the deposition, instead of being a 
film of solid copper on the cast, will be in the form of small 
grains on the lower end of the wire. The weakest power 
consistent with precipitation should therefore be applied. 

ELECTRO-GILDING. 

Gilding without a Battery .—After the solution is prepared, 
the process of electrotype gilding is quite simple, and may 
be performed by any one of common ingenuity. 

The solution for this purpose is cyanide of gold dissolved 
in pure water. This is prepared by dissolving the m^tal in 
aqua-regia, composed one part nitric, and two of muriatic 
acid. Ten, or fifteen grains of gold, to an ounce and a half 
of the aqua-regia, "may be the proportions. The acid being 
evaporated, the salt which is called the chloride of gold is 
dissolved in a solution, made by mixing an ounce of the 
cyanuret of potash with a pint of pure water. The cyanu¬ 
ret of potash is decomposed and a cyanide of gold remains 
in solution. About 20 grains of the chloride of gold is a 
proper quantity for a pint of the solution. The cyanuret of 
potash, and the chloride, or oxide of gold, may be bought at 
the apothecaries. 

Having prepared the solution, the most simple method of 
gilding is to pour a quantity of it into a glass jar, or a tum¬ 
bler, and place in it the silver, copper, or German silver to be 
gilded, in contact with a piece of bright zinc, and the pro¬ 
cess will immediately begin. No other battery, except that 
formed by the zinc, and metal which receives the gold, is re¬ 
quired. The zinc at the point of contact must be bright and 
well fastened to the other metal by a string or otherwise. 
The process will be hastened by warmth, which may be ap¬ 
plied by placing the jar and its contents in a vessel of warm 
water. So far as the author knows, this simple process 
originated with himself, and answers admirably as an experi- 


ELECTRO-MAGNETISM. 


349 

ment in the electrotype art. The gold, however, is apt to 
settle upon the zinc, but which may be prevented by a little 
shell-lac varnish rubbed on it, except at the point of contact. 
The handles of scissors, silver spectacles, pencils, &c., ma.y 
be handsomely gilt by this process. 

Gilding with a Battery .—If the operator desires to extend 
his experiments in the art of electro-gilding, a small battery 
must be employed, of which there are many varieties. The 
best for more extensive operations, is that composed of pla¬ 
tinized silver, and amalgamated zinc. 

For this purpose the platina is first dissolved in aqua-regia, 
in proportion of 10 grains to the ounce, and then precipitated 
on the silver. The silver is in sheets, such as is used for pla¬ 
ting, no thicker than thin writing paper. This may be ob¬ 
tained of the silver platers, and being well cleaned, is ready 
for the process. 

These plates being covered with platina, are insoluble in 
the acid employed, and hence they will last many years. 
The amalgamated plates are also durable, and do not require 
cleaning. 

These platinized sheets are confined between two plates 
of amalgamated zinc. The process of amalgamation con¬ 
sists in rubbing mercury, with a little mass of cotton wool 
held in the fingers, on the clean zinc. These plates may be 
fixed half an inch apart by means of little pieces of wood, 
with the sheets between them, but not touching each other. 
The plates, having a metallic connection, form the positive 
side of the battery, while a copper wire soldered to the silver 
sheet makes the negative side. The dimensions of these 
plates may be four or five inches long, and three or four wide. 

For experimental purposes, however, a less expensive bat¬ 
tery may be used, that represented by fig. 255, made of cop¬ 
per and zinc, being sufficient. 

To gild by means of a battery, place the solution, made 
as above described, in a glass vessel, and connect the article 
to be gilded with the pole coming from the zinc side of the 
battery, letting the other wire, which should be tipped with a 
little piece of gold, dip into the solution. The gilding pro¬ 
cess will immediately begin, and in three or four hours a good 
coat of gold will be deposited on the article immersed. 

To keep the solution quite pure, the tips of the poles 
where they dip into the fluid should be of gold. If they are 
of copper, a portion of the metal will be dissolved and in¬ 
jure the result. 

30 


350 


ELECTRO-MAGNETISM. 


ELECTRO-PLATIN G. 

The process of silvering copper, or the alloys of the metals, 
such as German silver, is done on the same principle as that 
described for gilding, but there seems to be more difficulty in 
making the process succeed to the satisfaction of the artist 
than there is in depositing gold. 

The following is the method employed by Mr. Sumner 
Smith, of this city, the most experienced electrotype artist 
within our acquaintance. It will succeed perfectly in the 
hands of those who will follow the directions. 

Make a solution of cyanuret of potash in pure water, in 
the proportion of an ounce to a pint. Having placed it in a 
glass vessel, prepare the battery for action as usual. Then 
attach to the pole of the silver, or copper side of the battery, 
a thin plate of silver, and immerse this in the cyanuret solu¬ 
tion. The pole from the zinc side being now dipped into the 
fluid, the electro-chemical action on the silver plate instantly 
begins, and a rapid decomposition of the metal is effected, 
and in a short time the solution will be saturated with the 
silver, as will be indicated by the deposition of the metal on 
the end of the copper pole coming from the zinc side of the 
battery. The solution is now ready for use, but the remains 
of the silver, still undissolved, must not be removed before 
immersing the articles to be plated, since the solution is thus 
kept saturated. 

This solution is much better than that prepared by dis¬ 
solving the silver separately in an acid, and then re-dissolving 
in the cyanuret of potash as is usually done, for in the latter 
case the silver is apt to be deposited on German silver, brass, 
iron and other metals, without the galvanic action, in which 
case it does not adhere well, whereas the solution made as 
above directed is not liable to this imperfection. 

During the preparation of the fluid, only a very small cop¬ 
per wire should be employed on the zinc side of the battery. 

The articles to be plated must be well cleaned before im¬ 
mersion. To effect this, dip them into dilute sulphuric acid 
for a few minutes, then rub them with sand or whiting, and 
rinse in pure water. 

Now having exchanged the small copper pole of the zinc^ 
side of the battery, for a larger one of the same metal, tipped 
with silver, connect the article to be plated with this, the 
other pole with the silver plate attached being still immersed 
in the solution. 


ELECTRO-MAGNETISM. 


351 


The process must now be watched, and the silver attached 
to the copper side raised nearly out of the fluid, in case bub¬ 
bles of hydrogen are observed to rise from the pole on the 
other side, or the articles attached to it. The greater the sur¬ 
face of silver in the fluid, the more energetic will be the ac¬ 
tion, short of the evolution of hydrogen from the other pole, 
but when this is observed, the decomposing silver must be 
raised so far out of the fluid as to stop its evolution. 

By this method, a thick and durable coat of silver may be 
placed on old copper tea-pots, candlesticks, or other vessels 
of this sort, where the silvering has been worn off by long 
use. 


PHOTOGRAPHY. 

The word, photography , means written, or delineated by 
light, and is descriptive of the manner in which the pictures, 
or designs we are about to describe are taken. The principle 
on which this art is founded is quite simple, and will be readily 
understood by those who have made chemical experiments, 
and especially with nitrate of silver, of which the common 
marking ink is made. This is merely a solution of some salt 
of silver, the nature of which is, to grow dark on exposure to 
light, but remains colorless when kept in a perfectly dark 
place. 

Now if a sheet of white paper be imbued with a solution 
of this salt, and then with the hand placed upon it, exposed 
to the light, there will be a figure of the hand left on the 
paper, in white, the ground being black. The reason of this, 
from what we have already said, is obvious ; that portion of 
the paper which is protected by the hand remains white, 
while that which is exposed to the light turns black. 

The photographic art consists in first covering common 
writing paper with the salt of silver, then taking the picture 
by means of the camera obscura, and afterwards applying 
some solution which prevents the ground from changing its 
color by exposure to the light. 

The chief difficulty lies in perfecting the latter part of the 
process, and for this purpose, as well as with respect to the 
particular salt of silver to be used, and the way of applying 
it, a great variety of methods have been devised. 

Nitrated Paper .—The most simple kind of photographic 
* paper is made by dissolving one ounce of the crystallized 
nitrate of silver in four ounces of pure water, and applying 
it to the paper by means of a soft brush. 


ELECTR0-MAGNETISA1. 


3 

For this purpose the paper must be fastened to a piece of 
board with pins at each corner. In putting on the solution 
care must be taken not to touch the same part twice with the 
brush, for if it is not spread equally, the sheet will grow 
darker in some parts than in others. 

The paper being dried by the fire of a darkened room, is 
then ready to receive the impression in the camera obscura. 
It is then soaked for a few minutes in warm water, by which 
the nitrate around the picture is washed away, and the paper 
will remain white. This is to be very carefully done, with 
the paper pinned to the board, otherwise it will be torn and 
spoiled. 

The nitrated paper, after being dried, and before the picture 
is taken, will become much more sensitive to the light if it is 
soaked in a solution of ising-glass, or rubbed over with the 
white of an egg. It is better, however, to do this before the 
nitrate of silver is put on. 

The paper prepared in this manner is not sufficiently sen¬ 
sitive to be changed by diffused light, and consequently re¬ 
quires the rays of the sun in order to produce the photogra¬ 
phic effect. 

Murio-nitrated Paper .—Another method of preparing the 
paper is first to moisten it with a solution of muriate of soda, 
(common salt) and then apply the nitrate of silver. 

For this experiment, dissolve fifty grains of the salt in an 
ounce of water, and soak the paper in the solution. For this 
purpose it must be pinned to a board as formerly directed. 
After being pressed with a linen cloth or with blotting paper, 
and thus dried, it is then twice washed with a solution made 
by dissolving one hundred and twenty grains of crystallized 
nitrate of silver in an ounce of rain water. It must be 
dried by the fire of a darkened room between each washing. 

This paper is very sensitive, the color changing by small 
degrees of light. It must therefore be kept in the dark to 
the moment of using. 

A great variety of other methods of making photographic 
paper are described in treatises on the art, and to those we 
must refer the student who is inquisitive on such subjects. 

Camera Obscura .—An instrument of this kind of the ordi¬ 
nary construction, has already been figured and described, 
but a more simple and less expensive apparatus will answer 
for experiments in the art under consideration. 

Any one who lives near a joiner’s shop, and who is desi- 


ELECTROMAGNETISM. 


353 


rous of making photographic experiments, can make his own 
camera obscura. 

For this purpose, two boxes, each a foot long and eight or 
ten inches square, the one sliding within the other, is all that 
is required for the body of the camera. In one of the boxes 
is placed the lens, an inch and a half, or two inches in diameter 
having a focal distance of 12 or 15 inches. The boxes are 
to be painted black on the inside to prevent the diffusion of 
light. This may be done with spirits of turpentine and 
lampblack. 

The paper is fastened to a piece of thin board, which is to 
be attached to the inner, or sliding box. Through the upper, 
and back part of the box, there is a small hole through which 
the operator can see to adjust the paper in the focus of the 
lens, by sliding the box in, or out, as the case requires. 

Taking care to turn the sensitive side of the paper towards 
the lens, place it so that the most defined images of things 
fall upon its surface. In this position it must remain a suffi¬ 
cient length of time to receive the impression. 

The time required for this is of course quite variable, de¬ 
pending on the intensity of the light and the sensibility of 
the paper. It may however be stated, as a general guide, 
that highly sensitive paper, in the sunshine of a summer 
morning, requires about thirty minutes for the impression to 
be complete. 

If the light is less intense and the paper less perfect, it 
ought to remain an hour in the camera. 

Fixing the Picture .—When the paper is made sensitive by 
the murio-nitrate of silver, as in the last process described, 
the picture is fixed, and the other parts of the paper rendered 
insensible by a solution of hyposulphite of soda. The solu¬ 
tion is made by dissolving an ounce of the salt in a quart of 
water. A portion of this being placed in a shallow dish, the 
pictures are introduced one at a time, and allowed to remain 
two or three minutes. They are then washed in pure water, 
and then may be dried by exposure to the sun, which now 
effects no change in the color. 

DAGUERREOTYPE. 

This branch of photography was the invention of M. Da¬ 
guerre, an ingenious French artist, and is entirely independ¬ 
ent of the art of taking impressions on paper, as above de¬ 
scribed. In that, the pictures are reversed, in this they are in 
the natural position, and instead of paper, the picture is on silver 
30* 


354 


ELECTRO-MAGNETISM. 


As an art, this is one of the most curious and wonderful 
discoveries of the present age; for when we witness the va¬ 
riety of means necessary to the result, it would appear equally 
improbable that either accident, or design, could possibly 
have produced such an end by means so various and com¬ 
plicated, and to which no other art, (save in the use of the 
camera obscura,) has the least analogy in the manner in 
which the object is accomplished. # 

This being a subject of considerable public interest, and 
withal, a strictly philosophical art, we shall here describe all 
the manipulations as they succeed each other in producing 
the result, a human likeness. 

The whole process may conveniently be divided into eight 
distinct operations. 1st. Polishing the plate. 2d. Exposing 
it to the vapor of iodine. 3d. Exposing it to the vapor of 
bromine. 4th. Adjusting the plate in the camera obscura. 
5th. Exposing it to the vapor of mercury. 6th. Removing 
the sensitive coating. 7th. Gilding the picture. 8th. Color¬ 
ing the picture. 

1. Polishing the Plate .—The plates are made of thin sheets 
of silver, plated on copper. It is said that for some unknown 
reason the photographic impression takes more readily on 
these plates, than on entire silver. The silver is only thick 
enough to prevent reaching the copper in the process of 
scouring and polishing. 

The polishing is considered one of the most difficult and 
important manipulations in the art, and hence hundreds of 
pages have been written to describe the various methods de¬ 
vised and employed by different artists or amateurs. 

We can only state here, that the plate is first scoured with 
emery to take off the impressions of the hammer in planish¬ 
ing ; then pumice, finely powdered, is used, with alcohol, to 
remove all oily matter, and after several other operations, it 
is finally given the last finish by means of a velvet cushion 
covered with rouge. 

2. Iodizing the Plate .—After the plate is polished, it is in¬ 
stantly covered from the breath, the light, and the air, nor must 
it be touched, even on the edges, with the naked hand ; but- 
being placed on a little frame, with the face down, it is carried 
to a box containing iodine, over which it is placed as a cover. 
Here it remains for a moment or two in a darkened room, 
being often examined by the artist, whose eye decides by the 
yellowish color to which the silver changes, the instant when 
the metal has combined with the proper quantity of iodine 


ELECTRO-MAGNETISM. 


355 


This is a very critical part of the process, and requires a 
good eye and much experience. The vapor of iodine forms 
a film of the iodide of silver on the metal, and it is this which 
makes it sensible to the light of the camera, by which the 
picture is formed. If the film of iodine is too t hick, the pic¬ 
ture will be too deep, and dark; if too thin, either a light 
impression, or none at all will be made. 

3. Exposure of the Vapor of Bromine. —Bromine is a pe¬ 
culiar substance, in the liquid form, of a deep red color, ex¬ 
ceedingly volatile, very poisonous, and having an odor like 
chlorine and iodine, combined. It is extracted from sea wa¬ 
ter, and the ashes of marine vegetables. 

This the photographic artists call an accelerating substance, 
because it diminishes the time required to take the picture 
in the camera obscura. 

The iodized plate will receive the picture without it, but 
the sitter has to remain without motion before the camera 
for several minutes, whereas by using the bromine, the im¬ 
pression is given, in a minute, or in a minute and a quarter. 
Now as the least motion in the sitter spoils the likeness, it is 
obvious that bromine is of much importance to the art, espe¬ 
cially to nervous people and children. 

The bromine is contained in a glass vessel closely covered, 
and is applied by sliding the plate over it for a few seconds. 

4. Adjusting the Plate in the Camera .—The plate is now 
ready for the photographic impression by means of the ca¬ 
mera. If the likeness of a person is to be taken, he is already 
placed before the instrument, in a posture which the artist 
thinks will give the most striking picture, and is told that the 
only motion he can make for a half a minute to a minute, is 
winking. 

The artist now takes the plate from a dark box, and un¬ 
der cover of a black cloth fixes it in the focus of the lens. 
This is done in a light room, with the rays of the sun dif¬ 
fused by means of white curtains. 

The artist having left the sitter for the specified time, re¬ 
turns, and removes the plate for the next operation. Still, 
not the least visible change has taken place on the bright 
surface of the silver. If examined ever so nicely, no sign of 
a human face is to be seen, and the sitter who sees the plate, 
and knows nothing of the art, wonders what next is to be 
done. 

5. Exposure to the fumes of Mercury. —The plate is next 
exposed to the fumes of mercury. This is contained in an 


356 


ELECTRO-MAGNETISM. 


iron oox in a darkened room, and is heated by means of an 
alcohol lamp, to about 180 degrees, Fah. The cover of the 
box being removed, the plate is laid on, with the silver side 
down, in its stead. 

After a few minutes, the artist examines it, and by a faint 
light, now sees that the desired picture begins to appear. It 
is again returned for a few minutes longer, until the likeness 
is fully developed. 

If too long exposed to the mercury, the surface of the silver 
turns to a dark ashy hue, and the picture is ruined ; if re¬ 
moved too soon, the impression is too faint to be distinct to 
the eye. 

6. Removal of the sensitive coating. —The next operation 
consists in the removal of the iodine, which not only gives 
the silver a yellowish tinge, but if suffered to remain, would 
darken, and finally ruin the picture. Formerly this was done 
by a solution of common salt, but experiment has shown that 
the peculiar chemical compound called hyposulphite of soda , 
answers the purpose far better. This is a beautiful, trans¬ 
parent crystallized salt, prepared by chemists for this express 
purpose. 

A solution of this is poured on the plate until the iodine is 
entirely removed, and now the picture for the first time may 
be exposed to the light of the sun without injury, but the 
plate has still to be washed in pure water, to remove all re¬ 
mains of the hyposulphite, and then heated and dried over an 
alcohol lamp. 

7. Gilding the Picture. —This is called fixing by the chlo¬ 
ride of gold. 

Having washed the picture thoroughly, it is then to be 
placed on the fixing stand, which is to be adjusted previous¬ 
ly, to a perfect level, and as much solution of chloride of 
gold as the plate can retain, poured on. The alcohol lamp 
is then held under all parts of it successively. At first the 
image assumes a dark color, but in a few minutes grows 
light, and acquires an intense, and beautiful appearance. 

The lamp is now removed, and the plate is again well 
washed in pure water, and then dried by heat. 

Before gilding, the impression may be removed by re¬ 
polishing the plate, when it is perfectly restored; but after 
gilding, no polishing or scouring will so obliterate the picture, 
as to make it answer for a second impression. Such plates 
are either sold for the silver they contain, or are re-plated by 
the electrotype process. 


ELECTROMAGNETISM. 


357 


8. (Joinring the Picture .—Coloring Daguerreotype pictures 
is an American invention, and has been considered a secret, 
though at the present time it is done with more or less suc¬ 
cess by most artists. 

The colors consist of the oxides of several metals, ground 
to an impalpable powder. They are laid on in a dry state, 
with soft camel hair pencils, after the process of gilding. 
The plate is then heated, by which they are fixed. This is 
a very delicate part of the art, and should not be undertaken 
by those who have not a good eye, and a light hand. 

The author is indebted to Mr. N. G. Burgess, of 192, 
Broadway, New York, for much of the information contain¬ 
ed in the above account of the Daguerreotype art. Mr. B. is 
an experienced and expert artist in this line. 


ELECTRO-MAGNETIC ALARM BELL. 

This little arrangement is employed in some of the large 
establishments in England, for the purpose of communicating 
intelligence from one part of the building to another. When 
the building is many stories high, and communication is to 
be made to workmen, or others, at the horizontal distance of 
several hundred feet, the usual fixtures of wire and yokes is 
both expensive and troublesome. In such cases this alarm 
bell is highly convenient, and sufficiently simple. Any one 
of common ingenuity may construct it by attending to the 
following cut, fig. 257, and description. 

The horse-shoe mag¬ 
net, a a , of soft iron, is 
supported by being let 
into a piece of board. 

This is wound with in¬ 
sulated copper wire, in 
the manner explained 
under the article “ Tem¬ 
porary. Magnets,” figure 
254. The armature, 6, 
also of soft iron, is sus¬ 
pended to the yoke, c, so 
as to play up and down. 

The wire d is connected 
with the yoke at one end, 
and with the bell ham¬ 
mer, e, at the other. The 

bell may be a common house bell, with the handle reversed, 
and fixed to the platform with the magnet. The letters g 
















353 


ELECTROMAGNETISM. 


and h show two little cups of mercury, fixed to the ends of 
the wire which surrounds the magnet. Into these cups are 
dipped the two poles of a small electrical battery when an 
alarm, or call, is to be made. 

This arrangement being made, all that is required to give 
an alarm at any distance to which the wires reach, is to dip 
one of the poles of the battery into the cup opposite to that 
into which the other pole is immersed. The bell and appa¬ 
ratus described, being of course in the room where the call is 
to be made, and the battery in the room from whence the call 
proceeds, with common copper bell wires extending from one 
to the other. On making the connection, the following effect 
is produced. The soft iron instantly becoming a strong 
magnet, attracts the armature, b , by which the hammer e is 
raised, giving a smart blow against the bell. On breaking 
the connection, the hammer falls, and in an instant will give 
another blow, on again dipping the pole into the cup. Thus 
signals may be given, consisting of any number of strokes 
agreed on. 

MORSE’S ELECTRO-MAGNETIC TELEGRAPH. 

The means by which Mr. Morse has produced his wonder¬ 
working and important machine is the production of a tem¬ 
porary magnet, by the influence of the galvanic fluid. 

We have already described the method of making tempo¬ 
rary magnets of soft iron, by covering the latter with insola- 
ted copper wire, to each end of which the poles of a small 
galvanic battery is applied. 

The description of fig. 254, with what is said before, on the 
subject, will inform the student how the power is obtained by 
which the philosopher in question has brought before the 
world such wonderful and unexpected effects. 

It has long since been known, that so far as experiment 
has taught, there is no appreciable time occupied in the pas¬ 
sage of the electric fluid from one place to another, though 
Morse’s experiments tend to prove, that for the first ten miles, 
there is a diminution of the magnetic power, after which, to 
the distance of 33 miles, no such effect is perceptible. 

The machine itself is sufficiently simple, and will be com¬ 
prehended at once, by those who have made electro-magnetic 
experiments, by the annexed diagram and description. 

The temporary magnet a, fig. 258, enveloped with its in- 
solated copper wire, is fastened to the wooden frame 6, g, by 
means of cords or otherwise. 


ELECTROMAGNETISM. 


359 


Fig. 258. 



This frame also supports the standard A, which sustains 
the revolving drum f on which the paper to receive the em¬ 
blematical alphabet is fixed, m being the edge of the paper. 

To the arm g, is appended the lever c, of wood, which has 
a slight vertical motion, in one direction by the steel spring 
dj and in the other, by the armature of soft iron e. 

The two poles of the magnet rest in two little cups of mer¬ 
cury, into which are also to be plunged the poles of the mag¬ 
netic battery, (not shown in the drawing,) of which p is the 
positive, and n the negative. The steel point i, attached to 
the lever, is designed to mark the telegraphic alphabet on the 
paper. 

Having thus explained the mechanism, we will now show 
in what manner this machine acts to convey intelligence 
from one part of the country to another. 

It has already been explained that when a bar of soft iron 
surrounded by insolated copper wire, as shown at a, has its 
two poles connected with the poles of a galvanic battery, the 
iron instantly becomes a magnet, but returns to its former 
state, or ceases to be magnetic, the instant the connection 
between them ceases. 

To break the connection, it is not necessary that both of 
the poles should be detached, the circuit being broken by the 
separation of one only. 

Supposing then, that a and p are the poles of such a bat¬ 
tery, on placing n into the cup of mercury, the wires from 
the soft iron being already there, the armature e is instantly 
attracted, which brings the point i against the paper on the 





















360 


ELECTRO-MAGNETISM. 


revolving wheel f If n is instantly detached after the point 
strikes the paper, then only a dot will be made, for the mag¬ 
netic power ceasing with the breaking of the circuit, the 
spring d, withdraws the point from the paper the instant the 
pole is removed. 

If a line is required in the telegraphic alphabet, then the 
pole is kept longer in the vessel of mercury, and as the al¬ 
phabet consists of dots, and lines of different lengths, it is 
obvious that writing in this manner cannot be difficult. The 
understanding of the alphabet is another matter, though we 
are informed that this may be done with facility. 

The marks of the point i, are made by indenting the pa¬ 
per, the roller on which it is fixed being made of steel in 
which a groove is turned, into which the paper is forced by 
the point. The paper is therefore raised on the under side 
like the printing for the blind. 

The roller / is moved by means of clock work, having a 
uniform motion, consequently the dots and lines depending 
on the time the point is made to touch the paper, are always 
uniform. 

Now with respect to the distance apart at which the tem¬ 
porary magnet and writing apparatus, and the battery are 
placed, experiment shows that it makes little difference with 
respect to time. Thus, suppose the battery is in Washing¬ 
ton, and the magnet in Baltimore, with copper wires reach¬ 
ing from one to the other. Then the telegraphic writer at 
Washington, giving the signal by means of an alarm bell, 
that he is ready to communicate, draws the attention of the 
person at Baltimore to the apparatus there,—the galvanic 
action being previously broken by taking one of the poles 
from the battery at Washington. 

If now we suppose the letter a is signified by a single dot, 
he at Washington dips the pole in the cup of the battery, 
and instantly at Baltimore the soft iron becomes a magnet, 
and a dot is made on the paper, and so, the rest of the al¬ 
phabet. 

The wires are carried through the air by being run 
through tubes elevated twenty feet from the ground. 


LRpFe'15 






























